SPERMATOPHYTES 
251 
often two regions of mesophyll (the palisade region of vertically elon- 
gated and close-lying cells, and the spongy region of rounded and 
loosely packed cells, leaving a labyrinth of intercellular spaces); and 
the veins, of varying order (fig. 559, v), which traverse the mesophyll 
and contain the vascular strands (conducting system) connecting with 
FIGS. 560, 561. 560, surface view of epidermis of leaf of lily, showing epidermal cells 
and stomata; 561, a single stoma more highly magnified. After COULTER. 
those of the stem and root, and also strands of mechanical or supporting 
tissue. Such a structure provides protection (epidermal layers) for the 
mesophyll cells, an internal atmosphere bathing the mesophyll cells 
and communicating with the external atmosphere through the stomata, 
a conducting system, and a mechanical framework. 

FLOWER 
General character. The flower is a very characteristic structure of 
angiosperms, but it is impossible to define it with exactness, so as to 
apply to all angiosperms and to no other group. In passing from 
gymnosperms to angiosperms there is a gradual transition from the 
structure called a strobilus to that called a flower or an inflorescence. 
The most characteristic feature of the flower of angiosperms is the pres- 
ence of a perianth associated with the sporophylls. In its full expres- 
