56 BOTANY. 
are intercellular and which at no time were composed of 
cells. In some cases they so closely resemble the cavities 
derived from cells that it is with the greatest difficulty 
that their real nature can be made out. In their simplest 
form they are the small irregular spaces which appear 
during the rapid growth of parenchyma-cells (Fig. 35); 
from these to the large regular canals which are common 
in many water-plants there are all intermediate gradations. 
Bee ge neni eerie rere eerie tee 
i dale Mae weet caused by splitting of the walls during rapid 
107. In leaves, especially in the soft tissue of the under 
portion, there are usually many large irregular spaces be- 
tween the cells; they are in communication with the exter- 
nal air through the breathing-pores, and contain only air 
and watery vapor. The leaf-stalks and stems of many 
aquatic plants contain exceedingly large air-conducting 
intercellular canals, which occupy even more space than 
the surrounding tissues (Fig. 36). In the rushes, water- 
lilies, and water-plantains they are so large as to be readily 
seen by the naked eye. These all are in communication 
