75^ 
.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 Pandorineae 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 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 Impaiiens) 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. für 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 Wärme auf Schw^ärmsporen, Jen. Zeitschr. XII, 1878; Stahl, Ueb. den Einfluss des 
Lichts auf die Bewegung der Schwärmsporen, Bot. Zeitg. 1878, and Verh. d, phys.-med. Gesellsch. in 
Würzburg, 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.] 
^ 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. 
