It has been stated that some of the leaflets experimented on were fastened close to the cork, and others at a height of from to 3/4 of an inch above it; and that whenever, after exposure to a frost, any difference could be detected in their states, the closely pinned ones had suffered most. We attributed this difference to the air, not cooled by radiation, having been prevented from circulating freely beneath the closely pinned leaflets. That there was really a difference in the temperature of leaves treated in these two different methods, was plainly shown on one occasion; for after the exposure of a pot with plants of Melilotus dentata for 2 h. to a clear sky (the temperature on the surrounding gra.s.s being -2o C.), it was manifest that more dew had congealed into h.o.a.r-frost on the closely pinned leaflets, than on those which stood horizontally [page 296]
a little above the cork. Again, the tips of some few leaflets, which had been pinned close to the cork, projected a little beyond the edge, so that the air could circulate freely round them. This occurred with six leaflets of Oxalis acetosella, and their tips certainly suffered rather less then the rest of the same leaflets; for on the following morning they were still slightly green. The same result followed, even still more clearly, in two cases with leaflets of Melilotus officinalis which projected a little beyond the cork; and in two other cases some leaflets which were pinned close to the cork were injured, whilst other free leaflets on the same leaves, which had not s.p.a.ce to rotate and a.s.sume their proper vertical position, were not at all injured.
Another a.n.a.logous fact deserves notice: we observed on several occasions that a greater number of free leaves were injured on the branches which had been kept motionless by some of their leaves having been pinned to the corks, than on the other branches. This was conspicuously the case with those of Melilotus Pet.i.tpierreana, but the injured leaves in this instance were not actually counted. With Arachis hypogaea, a young plant with 7 stems bore 22 free leaves, and of these 5 were injured by the frost, all of which were on two stems, bearing four leaves pinned to the cork-supports.
With Oxalis carnosa, 7 free leaves were injured, and every one of them belonged to a cl.u.s.ter of leaves, some of which had been pinned to the cork.
We could account for these cases only by supposing that the branches which were quite free had been slightly waved about by the wind, and that their leaves had thus been a little warmed by the surrounding warmer air. If we hold our hands motionless before a hot fire, and then wave them about, we [page 297]
immediately feel relief; and this is evidently an a.n.a.logous, though reversed, case. These several facts--in relation to leaves pinned close to or a little above the cork-supports--to their tips projecting beyond it-- and to the leaves on branches kept motionless--seem to us curious, as showing how a difference, apparently trifling, may determine the greater or less injury of the leaves. We may even infer as probable that the less or greater destruction during a frost of the leaves on a plant which does not sleep, may often depend on the greater or less degree of flexibility of their petioles and of the branches which bear them.
NYCt.i.tROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.
We now come to the descriptive part of our work, and will begin with cotyledons, pa.s.sing on to leaves in the next chapter. We have met with only two brief notices of cotyledons sleeping. Hofmeister,* after stating that the cotyledons of all the observed seedlings of the Caryophylleae (Alsineae and Sileneae) bend upwards at night (but to what angle he does not state), remarks that those of Stellaria media rise up so as to touch one another; they may therefore safely be said to sleep. Secondly, according to Ramey**, the cotyledons of Mimosa pudica and of Clianthus Dampieri rise up almost vertically at night and approach each other closely. It has been shown in a previous chapter that the cotyledons of a large number of plants bend a little upwards at night, and we here have to meet the difficult question at what inclination may they be said to sleep? According to the view which we maintain, no movement deserves to be called
* "Die Lehre von der Pflanzenzelle," 1867, p. 327.
** "Adansonia," March 10th, 1869.
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nyct.i.tropic, unless it has been acquired for the sake of lessening radiation; but this could be discovered only by a long series of experiments, showing that the leaves of each species suffered from this cause, if prevented from sleeping. We must therefore take an arbitrary limit. If a cotyledon or leaf is inclined at 60o above or beneath the horizon, it exposes to the zenith about one-half of its area; consequently the intensity of its radiation will be lessened by about half, compared with what it would have been if the cotyledon or leaf had remained horizontal. This degree of diminution certainly would make a great difference to a plant having a tender const.i.tution. We will therefore speak of a cotyledon and hereafter of a leaf as sleeping, only when it rises at night to an angle of about 60o, or to a still higher angle, above the horizon, or sinks beneath it to the same amount. Not but that a lesser diminution of radiation may be advantageous to a plant, as in the case of Datura stramonium, the cotyledons of which rose from 31o at noon to 55o at night above the horizon. The Swedish turnip may profit by the area of its leaves being reduced at night by about 30 per cent., as estimated by Mr. A.
S. Wilson; though in this case the angle through which the leaves rose was not observed. On the other hand, when the angular rise of cotyledons or of leaves is small, such as less than 30o, the diminution of radiation is so slight that it probably is of no significance to the plant in relation to radiation. For instance, the cotyledons of Geranium Iberic.u.m rose at night to 27o above the horizon, and this would lessen radiation by only 11 per cent.: those of Linum Berendieri rose to 33o, and this would lessen radiation by 16 per cent.
There are, however, some other sources of doubt with [page 299]
respect to the sleep of cotyledons. In certain cases, the cotyledons whilst young diverge during the day to only a very moderate extent, so that a small rise at night, which we know occurs with the cotyledons of many plants, would necessarily cause them to a.s.sume a vertical or nearly vertical position at night; and in this case it would be rash to infer that the movement was effected for any special purpose. On this account we hesitated long whether we should introduce several Cucurbitaceous plants into the following list; but from reasons, presently to be given, we thought that they had better be at least temporarily included. This same source of doubt applies in some few other cases; for at the commencement of our observations we did not always attend sufficiently to whether the cotyledons stood nearly horizontally in the middle of the day. With several seedlings, the cotyledons a.s.sume a highly inclined position at night during so short a period of their life, that a doubt naturally arises whether this can be of any service to the plant. Nevertheless, in most of the cases given in the following list, the cotyledons may be as certainly said to sleep as may the leaves of any plant. In two cases, namely with the cabbage and radish, the cotyledons of which rise almost vertically during the few first nights of their life, it was ascertained by placing young seedlings in the klinostat, that the upward movement was not due to apogeotropism.
The names of the plants, the cotyledons of which stand at night at an angle of at least 60o with the horizon, are arranged in the appended list on the same system as previously followed. The numbers of the Families, and with the Leguminosae the numbers of the Tribes, have been added to show how widely the plants in question are distributed throughout the [page 300]
dicotyledonous series. A few remarks will have to be made about many of the plants in the list. In doing so, it will be convenient not to follow strictly any systematic order, but to treat of the Oxalidae and the Leguminosae at the close; for in these two Families the cotyledons are generally provided with a pulvinus, and their movements endure for a much longer time than those of the other plants in the list.
List of Seedling Plants, the cotyledons of which rise or sink at night to an angle of at least 60o above or beneath the horizon.
Bra.s.sica oleracea. Cruciferae (Fam. 14).
-- napus (as we are informed by Prof. Pfeffer). Rapha.n.u.s sativus.
Cruciferae.
Githago segetum. Caryophylleae (Fam. 26).
Stellaria media (according to Hofmeister, as quoted). Caryophylleae.
Anoda Wrightii. Malvaceae (Fam. 36).
Gossypium (var. Nankin cotton). Malvaceae.
Oxalis rosea. Oxalidae (Fam. 41).
-- floribunda.
-- articulata.
-- Valdiviana.
-- sensitiva.
Geranium rotundifolium. Geraniaceae (Fam. 47).
Trifolium subterraneum. Leguminosae (Fam. 75, Tribe 3).
-- strictum.
-- leucanthemum.
Lotus ornithopopoides. Leguminosae (Tribe 4).
-- peregrinus.
-- Jacobaeus.
Clianthus Dampieri. Leguminosae (Tribe 5)--according to M. Ramey.
Smithia sensitiva. Leguminosae (Tribe 6).
Haematoxylon Campechianum. Leguminosae (Tribe 13)--according to Mr. R. I.
Lynch.
Ca.s.sia mimosoides. Leguminosae (Tribe 14).
-- glauca.
-- florida.
-- corymbosa.
-- p.u.b.escens.
-- tora.
-- neglecta.
-- 3 other Brazilian unnamed species.
Bauhinia (sp.?. Leguminosae (Tribe 15).
Neptunia oleracea. Leguminosae (Tribe 20).
Mimosa pudica. Leguminosae (Tribe 21).
-- albida.
Cucurbita ovifera. Cucurbitaceae (Fam. 106).
-- aurantia.
Lagenaria vulgaris. Cucurbitaceae.
Cuc.u.mis dudaim. Cucurbitaceae.
Apium petroselinum. Umbelliferae (Fam. 113).
-- graveolens.
Lactuca scariola. Compositae (Fam. 122).
Helianthus annuus (?). Compositae.
Ipomoea caerulea. Convolvulaceae (Fam. 151).
-- purpurea.
-- bona-nox.
-- coccinea.
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List of Seedling Plants (continued).
Solanum lycopersic.u.m. Solaneae (Fam. 157.) Mimulus, (sp. ?) Scrophularineae (Fam. 159)--from information given us by Prof. Pfeffer.
Mirabilis jalapa. Nyctagineae (Fam. 177).
Mirabilis longiflora.
Beta vulgaris. Polygoneae (Fam. 179).
Amaranthus caudatus. Amaranthaceae (Fam. 180).
Cannabis sativa (?). Cannabineae (Fam. 195).
Bra.s.sica oleracea (Cruciferae).--It was shown in the first chapter that the cotyledons of the common cabbage rise in the evening and stand vertically up at night with their petioles in contact. But as the two cotyledons are of unequal height, they frequently interfere a little with each other"s movements, the shorter one often not standing quite vertically. They awake early in the morning; thus at 6.45 A.M. on Nov. 27th, whilst if was still dark, the cotyledons, which had been vertical and in contact on the previous evening, were reflexed, and thus presented a very different appearance. It should be borne in mind that seedlings in germinating at the proper season, would not be subjected to darkness at this hour in the morning. The above amount of movement of the cotyledons is only temporary, lasting with plants kept in a warm greenhouse from four to six days; how long it would last with seedlings growing out of doors we do not know.
Rapha.n.u.s sativus.--In the middle of the day the blades of the cotyledons of 10 seedlings stood at right angles to their hypocotyls, with their petioles a little divergent; at night the blades stood vertically, with their bases in contact and with their petioles parallel. Next morning, at 6.45 A.M., whilst it was still dark, the blades were horizontal. On the following night they were much raised, but hardly stood sufficiently vertical to be said to be asleep, and so it was in a still less degree on the third night.
Therefore the cotyledons of this plant (kept in the greenhouse) go to sleep for even a shorter time than those of the cabbage. Similar observations were made, but only during a single day and night, on 13 other seedlings likewise raised in the greenhouse, with the same result.
The petioles of the cotyledons of 11 young seedlings of Sinapis nigra were slightly divergent at noon, and the blades stood at right angles to the hypocotyls; at night the petioles were in close contact, and the blades considerably raised, with their bases in contact, but only a few stood sufficiently upright to be called asleep. On the following morning, [page 302]
the petioles diverged before it was light. The hypocotyl is slightly sensitive, so that if rubbed with a needle it bends towards the rubbed side. In the case of Lepidium sativum, the petioles of the cotyledons of young seedlings diverge during the day and converge so as to touch each other during the night, by which means the bases of the tripart.i.te blades are brought into contact; but the blades are so little raised that they cannot be said to sleep. The cotyledons of several other cruciferous plants were observed, but they did not rise sufficiently during the night to be said to sleep.
Githago segetum (Caryophylleae).--On the first day after the cotyledons had burst through the seed-coats, they stood at noon at an angle of 75o above the horizon; at night they moved upwards, each through an angle of 15o so as to stand quite vertical and in contact with one another. On the second day they stood at noon at 59o above the horizon, and again at night were completely closed, each having risen 31o. On the fourth day the cotyledons did not quite close at night. The first and succeeding pairs of young true leaves behaved in exactly the same manner. We think that the movement in this case may be called nyct.i.tropic, though the angle pa.s.sed through was small. The cotyledons are very sensitive to light and will not expand if exposed to an extremely dim one.
Anoda Wrightii (Malvaceae).--The cotyledons whilst moderately young, and only from .2 to .3 inch in diameter, sink in the evening from their mid-day horizontal position to about 35o beneath the horizon. But when the same seedlings were older and had produced small true leaves, the almost orbicular cotyledons, now .55 inch in diameter, moved vertically downwards at night. This fact made us suspect that their sinking might be due merely to their weight; but they were not in the least flaccid, and when lifted up sprang back through elasticity into their former dependent position. A pot with some old seedlings was turned upside down in the afternoon, before the nocturnal fall had commenced, and at night they a.s.sumed in opposition to their own weight (and to any geotropic action) an upwardly directed vertical position. When pots were thus reversed, after the evening fall had already commenced, the sinking movement appeared to be somewhat disturbed; but all their movements were occasionally variable without any apparent cause. This latter fact, as well as that of the young cotyledons not sinking nearly so much as the older ones, deserves notice.
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Although the movement of the cotyledons endured for a long time, no pulvinus was exteriorly visible; but their growth continued for a long time. The cotyledons appear to be only slightly heliotropic, though the hypocotyl is strongly so.
Gossypium arboreum (?) (var. Nankin cotton) (Malvaceae).--The cotyledons behave in nearly the same manner as those of the Anoda. On June 15th the cotyledons of two seedlings were .65 inch in length (measured along the midrib) and stood horizontally at noon; at 10 P.M. they occupied the same position and had not fallen at all. On June 23rd, the cotyledons of one of these seedlings were 1.1 inch in length, and by 10 P.M. they had fallen from a horizontal position to 62o beneath the horizon. The cotyledons of the other seedling were 1.3 inch in length, and a minute true leaf had been formed; they had fallen at 10 P.M. to 70o beneath the horizon. On June 25th, the true leaf of this latter seedling was .9 inch in length, and the cotyledons occupied nearly the same position at night. By July 9th the cotyledons appeared very old and showed signs of withering; but they stood at noon almost horizontally, and at 10 P.M. hung down vertically.