THE CONDITIONS OF PLANT LIFE 21 
bearing leaves. This is one of many instances where 
in response to similar needs there has been a parallel 
development in groups which genetically are widely 
separated. 
It is, of course, among the Spermatophytes (seed- 
bearing or flowering plants) that the highest degree 
of specialization, both of the plant body and tissues, is 
reached. The plant usually shows a definite main axis 
or stem, to which aré attached a variety of appendicular 
organs — leaves, roots, and branches. The tissues of 
which thése various organs are composed show much 
variation in the cells of which they are made up. The 
green tissue is mainly restricted to the leaves, where 
it is so placed as to be most favorably situated 
with reference to the light. As a rule the outer or 
epidermal cells are not provided with chloroplasts, but 
serve as a protection for the delicate green cells lying 
below them, and in case the plant is exposed to great 
heat or dryness, the epidermal cells become much 
thickened and almost impervious to water, so that the 
loss of water from the green cells is effectively checked. 
Familiar examples of this kind are seen in the leaves 
of the laurel, oleander, and many other evergreens. 
As it is necessary, however, for the green cells to have 
communication with the atmosphere in order to obtain 
the necessary carbon dioxide and oxygen, this is pro- 
vided for by the development of the stomata or breath- 
ing pores always found upon the leaves, and these 
communicate with the numerous air-spaces between 
‘the green cells which are thus brought directly into 
contact with the atmospheric gases. 
Within the green cells the decomposition of the car- 
