ACTION OF LIGHT ON VEGETATION. 753 



I 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 Spirogyra, Vaucheria, 

 Hydrodictyon, Ulothrix, &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 Pilobolus cryslalltnus, 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 ammoniacal 

 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 cell-walls, cell-division still 

 taking place only occasionally or not at all. 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., Tetersburg 1867, vol. VI; Pfeffer, Arbeilen des bot, Inst, in 

 WUrzburg, vol. I, 1871, p. 80. 



3 c 



