20 PLANT LIFE 
with food materials, it multiplies rapidly. 
When an individual has reached a certain 
size it divides into two, and this process being 
repeated in the various individuals of a colony 
the Pleurococecus spreads rapidly over the 
surface of the damp wood. Furthermore, 
the individual plants withstand desiccation 
without dying, and in this condition they are 
carried by currents of air to fresh spots where 
new colonies can be started. 
But to return to Chlamydomonas. Its 
second feature of importance, from our present 
point of view, consists in its greenness. 
As we have seen, the green colouring matter, 
or chlorophyll, is not diffused through the whole 
protoplasm, but is restricted to one or more 
(in this plant, one) definite and specialised 
masses or corpuscles, each of which constitutes 
a chloroplast. The part which the chloroplast 
plays in the cell 1 is that of utilising the energy 
1 CeLL.—This is the term commonly used to denote the 
unit of a living organism, though, unfortunately, it is 
not always used in the same sense by different writers. 
In this book it will be taken to denote a mass of protoplasm 
(whether enclosed in a cellulose membrane or not) which 
ts dominated by a single nucleus. This protoplasmic mass 
is commonly, but not necessarily, separated by a cell 
membrane from other similar ones in the cell-aggregates 
which together constitute the bodies of larger ‘plants. 
A plant may thus consist of (1) a single cell; (2) a number 
of coherent cells, each more or less delimited from the 
rest by a membranous envelope or partition wall; (3) a 
number of coalescent cells, consisting of protoplasmic 
units, each containing one nucleus, but the units not 
separated from each other by cell walls. Such a cell colony 
