752 GENERAL CONDITIONS OF PLANT-LIFE. 



hyaline, and provided with cilia) becomes the base of the germinating plant when 

 the zoogonidium has come to rest. These movements of zoogonidia and the very 

 similar ones of the Pandorinese 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 indi- 

 vidual. Cohn states that here also the less refrangible rays have the same effect as 

 darkness, while the direction of the motion is determined by the blue and the 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 accompanied by 

 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 Aroidese 

 (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 de- 

 veloped 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- 



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

 questioned by Schmidt. [See Sachs, Ueb. Emulsionsfiguren, Flora, 1876; Strasburger, Wirkung des 

 Lichts und der Warme auf Schwarmsporen, Jen. Zeitschr. XII, 1878; Stahl, Ueb. den Einfluss des 

 Lichts auf die Bewegung der Schwarmsporen, Bot. Zeitg. 1878, and Verh. d. phys.-med. Gesellsch. in 

 Wurzburg, 1879. It appears that the zoogonidia place themselves so that their long axes coincide 

 with the direction of the incident rays. They move either towards the source of light or away from 

 it, the direction of their movement being dependent upon a number of conditions, such as the 

 intensity of the light, the relative temperature of different portions of the water in which the 

 zoogonidia are, the age of the zoogonidia, and the amount of oxygen in the water. Zoogonidia which 

 exhibit these phenomena are said, by Strasburger, to be phototactic. Some zoogonidia (such as those 

 of Saprolegnia) do not appear to be affected by light.] 



2 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. 



