ACTION OF LIGHT ON VEGETATION. 
753 
material are the bulbs, tubers, rhizomes, parts of the stem, cotyledons, and endo- 
sperm; after the complete exhaustion of these growth ceases in the dark but 
continues in the light, because the assimilating organs can then produce new 
material. This relation of growth which is connected with cell-division to assimi- 
lation is especially clear in Algae of simple structure (as Spirogjyra, Vaiicheria^ 
Hydrodiciyon, Ulolhrix, &c.), which assimilate in the day-time under the influence 
of light, while cell-division proceeds exclusively or at least chiefly at night. The 
swarm-spores are also formed in the night, but swarm only with access of daylight. 
In some Fungi also, as Piloholus crysiallimis, the splitting up of the protoplasm 
in the sporangium into a number of spores takes place only in the night, the spores 
being thrown out on access of light. While therefore in the larger and more 
highly organised plants assimilation and the construction of new cells out of the 
assimilated substances is carried on in different parts but at the same time, in small 
transparent plants in which the parts where these functions are effected are not 
surrounded by opaque envelopes they take place at different times. We have here a 
case of division of physiological labour which shows us that the cells which have to 
do with chemical work (assimilation) cannot at the same time perform the mecha- 
nical labour of cell-division; the two kinds of labour are distributed in the higher 
plants in space ; in very simple plants in time. Provided there is a supply of 
assimilated reserve-material, cell-division can therefore take place either in the 
light or the dark. Whether there are special cases in which light promotes or 
hinders cell-division is not known with certainty. We might suppose we have such 
a case when Fern-spores and the gemmae of Marchantia ^ germinate in the light 
but not in the dark ; but Borodin has shown that the less refrangible rays are 
alone active in this process of growth, mixed blue light (passed through am.moniacal 
copper oxide) acting like complete darkness. But since the less refrangible rays, as 
we have seen, have exactly the same effect on growth as the absence of light, but 
on the other hand are the efficient agent in assimilation, it may be supposed that 
these spores and gemmae do not contain certain substances necessary for germi- 
nation which must therefore be produced by assimilation. On the other hand it 
has not yet been explained on what depends the formation in long-continued dark- 
ness from many stems (as those of Cactus, Tropceolum, Hedera, &c.) of roots which 
are not produced under the ordinary amount of light. Whether the degree of 
humidity is an element in this is uncertain but not improbable. 
When the young organs emerge from the bud-condition, an active growth 
commences which is chiefly occasioned by the absorption of water into the cells 
and by a corresponding superficial extension of the ceU-walls, cell-division still 
taking place only occasionally or not at afl. This process of expansion takes 
place, in the case of aerial stems and foliar structures, in the daylight which 
penetrates deep into the transparent succulent tissues. In order to estimate the 
amount of its influence on these processes, it is best to grow seedlings or shoots 
of the same species of plant in continuous complete darkness, and others under 
an alternation of day and night, especially in the height of summer. Independently 
^ Borodin, Melanges biol., Petersburg 1867, vol. VI; Pfeffer, Arbeilen des bot. Inst, in 
Würzburg, vol. I, 1871, p. 80. 
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