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. i) 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 49, 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 whic'h 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 t>pes of parenchyma cells: 

 (i) 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, 



