ACTION OF LIGHT Oi\ VEGETATION. 673 



The swarming of zoospores is also connected with protoplasmic movements. 

 Their motile organs, the cilia, are supposed to be themselves only slender threads 

 of protoplasm, by the vibration of which both the rotatory and the advancing move- 

 ment of the zoospores is caused. The axis of rotation becomes subsequently the 

 axis of growth; the anterior end in the advancing motion (where the zoospore 

 is usually narrower, hyaline, and provided with cilia), becomes the base of the 

 germinating plant when the zoospore has come to rest. These movements of 

 zoospores and the very similar ones of Volvox are affected by light to this extent, 

 that when the light comes from one side they either tend towards or away from 

 the source of light, this depending apparently partly on the species and partly on 

 the age of the individual. Here also Cohn states that the less refrangible rays have 

 the same effect as darkness, while the direction of the motion is determined by the 

 blue and therefore more highly refrangible rays\ 



(e) Cell-Division and Growth''-'. The first formation and early growth of the 

 new organs in the higher plants consisting of masses of tissue is the result of a great 

 number of cell-divisions, which usually take place in complete darkness; as, for 

 example, in the roots of land and marsh-plants, the buds on underground rhizomes, 

 and leaves and flowers which are produced within the dense envelopes of the 

 bud. Cell-formation of the same kind may however take place under the influence 

 of light which may even be intense, as is shown by the growth of the roots of 

 land-plants in water exposed to light, or that of the aerial roots of Aroideae 

 (which are highly transparent at their cell-forming apex). The formation of 

 stomata and hairs which is the result of cell-division may take place either in 

 the light or in complete darkness within the bud, without any essential difference 

 being observable in the two cases. In the same manner the cambium of the 

 trunks of trees is covered by completely opaque envelopes, such as bark; while 

 that of many annual stems (as Impatiens) is exposed to the light which penetrates 

 the thin succulent cortex. Similar phenomena are presented in the formation and 

 ripening of ovules within transparent or completely opaque ovaries. They are most 

 obvious when shoots or even flowers which under ordinary circumstances are deve- 

 loped in the light are made to grow in complete darkness from bulbs, tubers, or 

 seeds. The small variations from the normal condition which occur in such cases 

 do not affect the early development of the organs; but their later growth which 

 does not depend on cell-division is necessarily interfered with, as well as the 

 development of chlorophyll. An obvious and necessary condition of these processes 

 of growth, whether in the dark or the light, is the presence of a supply of assimi- 

 lated reserve-materials, at the expense of which the formation of new cells can take 

 place. In the case of the buds of the higher plants their reservoirs of reserve- 

 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 



1 Cohn, Schles. Ges. fur vaterl. Cultur, Oct. 19, 1865. The facts have however recently been 

 questioned by Schmidt. 



^ Sachs, Ueber den Einfluss des Tageslichtes auf Neubildung u. Entfaltung verschiedener 

 Pflanzen-organe, Bot. Zeitg. 1863, Supplement. If I here consider cell-division and growth as 

 essentially mechanical processes, this does not imply that chemical changes do not also accompany 

 every process of growth. 



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