144 HISTOLOGY OF MEDICINAL PLANTS 
plants is meagre, still in other plants it is practically certain 
that the latex is composed of nutritive substances which are 
utilized by the plant as food. In certain other plants the latex 
appears to be used as a means of resisting insect attacks and as 
a protection against injury. 
There are two types of latex tubes common to plants, namely, 
latex cells and latex vessels. Latex tubes developing from a 
single cell do not differ materially from a latex tube originating 
from the fusion of several cells. In each case the latex tube 
branches to such an extent that it bears no resemblance to or- 
dinary cells. It would seem that the ultimate branches are formed 
and develop in much the same manner as root hairs—that is, 
by a growing tip of the branch. A mature plant may therefore 
have latex tubes with almost numberless branches (Plate 50, 
Fig. 1) and be of very great length. 
The branches of latex tubes develop in such an irregular 
manner that it is possible to obtain a cross and a longitudinal 
section of the latex tubes by making a cross-section of stem. 
Such a section is shown in the drawing of the cross-section of 
the rhizome of black Indian hemp (Plate 40, Fig. B). 
The color of the latex in medicinal plants varies from a 
gray white in papaw (carica papaya), aromatic sumac, black 
Indian hemp, and bitter root, to white in the opium poppy, 
light orange in celandine, and deep orange in bloodroot (Plate 
50, Fig. 2). In each of these cases it is the latex which yields 
the important medicinal products. 
PARENCHYMA 
The larger amount of plant tissue is composed of parenchyma 
cells. These cells vary from square to oblong, or they may be 
irregular and branched. The end walls are square or blunt, 
and the wall is composed of cellulose, with the exception of the 
wood parenchyma, which has lignified walls. 
There are seven characteristic types of parenchyma cells: 
(1) cortical parenchyma, (2) pith parenchyma, (3) wood par- 
enchyma, (4) leaf parenchyma, (5) aquatic plant parenchyma, 
(6) endosperm parenchyma, (7) phloem parenchyma. 
Parenchyma cells, cortical, pith, aquatic plant, leaf, flower, 
