90 



HISTOLOGY AND PHYSIOLOGY. 



psmdo-parmchyma (Fig. 204). lu prosenchyma the 

 cells are always elongated, and 

 have pointed ends wedged into one 

 another (Figs. 205, 209, pros). 

 204 When the cell-walls are thickened 



and excessively hard (Fig. 206), the tissue is designated as 

 sclerenchyma (Gr. scleras, hard). Beneath the epi- 

 dermis of some plants tissue is found, whose cell-walls are 

 thin, except at the corners, where they are thickened ; and 

 the word coUenchyma has been used to designate such 

 (Fig. 207). 



115. Vessels containing a milky 

 juice (latex) are called laticiferous 

 vessels (Fig. 208). Most of them 

 arise by cell-fusion, and may form a 

 net-work, penetrating the other tis- 

 sue. The walls are generally thicker 

 than that of the surrounding tissue. The latex is an emul- 

 sion of several substances. Some of these, as Caoutchouc 

 (India Eubber), Gutta-Percha, and Opium, are of great 

 economic importance. In Euphorbia elongated grains of 

 starch are found in the latex. Glands are secreting-cells, 

 or clusters of cells, with character- 

 istic contents, either odoriferous, acrid, 

 colored, oily, or resinous, which "find 

 no further use in changes connected 

 with nutrition or growth." The se- 

 cretion may collect in the interior of 

 the gland, as in case of Oil of Cam- 

 phor ; or it may be discharged exter- 



Fig. 204. Pseudo-parenchyma. Fig. 205. Prosencliyma of Indian Mallow {Adu~ 

 tiltm AvicenntE). Fig. 206. Sclerenchyma from a Pear. 



