316 
THE ZYGNEMACE/E. 
This is a form of the interstitial mode of growth. A full 
description of this process, as well as of the more complicated 
process of nuclear division, or Jcaryokinesis, will be found in 
Sachs’ “ Text book," last edition (1882), pp. 16-18. Whoever 
desires, however, to observe this phenomenon for himself 
must needs rise early, for it is a remarkable fact in the history 
of these plants that this process—as well as most other of 
the higher vital phenomena, or those concerned in repro¬ 
duction—takes place in the early dawn. I cannot here do 
better than give Sachs’ explanation for this curious fact. 
“ An obvious and necessary condition of these processes 
of growth, whether in the dark or the light, is the presence 
of the supply of assimilated 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-materials are the bulbs, tubers, rhizomes, parts of 
the stem, cotyledons, and endosperm ; 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 assimilation is especially clear in algae 
of simple structure (as Spiroyyra, etc.), which assimilate in the 
daytime 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. 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 physio¬ 
logical labour, which shows us that the cells which have to 
do with chemical work (assimilation) cannot at the same 
time perform the mechanical labour of cell-division ; the 
two kinds of labour are distributed in the higher plants in 
space, in very simple plants in time.” (“ Text book,” edition 
1882, pp. 752-3.) 
It seems highly probable that there is a specific limit to 
the power of extension by cell-division, and that when this 
is reached there comes into play that far more important 
operation which leads to the production of a new generation 
by the formation of a spore. And this conclusion would be 
quite in accordance with analogy, as a flowering plant usually 
only produces seed when the vigour of its vegetative growth 
is waning. 
