16 BOTANY. 
most important and the most variable. It is composed of 
cells whose walls are thin, colorless, or nearly so, and trans- 
parent; in outline they may be rounded, cubical, polyhe- 
dral, prismatic, cylindrical, tabular, stellate, and of many 
other forms. When the cells are bounded by plane sur- 
faces, generally, but not always, the end planes lie at right 
angles to the longer axis of the cells. 
37. This tissue makes up the whole of the substance of 
many of the lower plants, while in the higher it composes 
: the essential portions of the as- 
similative (green), vegetative 
(growing), and reproductive 
parts. 
88. Thick - angled Tissue 
(Collenchyma).—The cells of 
this tissue are elongated, usu- 
ally prismatic, and their trans- 
verse walls are most frequent- 
ly horizontal, rarely inclined. 
The walls are greatly thick- 
= ened along their longitudinal 
Fra. 8,—Cross-section of thick-angled gn gles, while the remaining 
tissue (cl) of Begonia petiole, showing : : 
the thickened angles. e, epidermis; parts are thin (Fig. 8). The 
chl, chlorophyll-bodies. Magnified 550 
ra cells contain chlorophyll, and 
retain the power of fission. Wet specimens show by trans- 
mitted light a characteristic bluish-white lustre, which is 
best seen in cross-sections. 
39. Thick-angled tissue is found beneath the epidermis 
of most flowering plants (and some ferns), usually as a mass 
of considerable thickness, and is doubtless developed from 
soft tissue for the purpose of giving support and strength 
to the epidermis. 
