90 



mSTOL OGY AND PHYSIOL OGY. 



pseudo-parenchyma (Fig. 204). lu prosenchyma the 

 Aj^^_^_^^_^^ cells are always elongated, and 

 have pointed ends wedged into one 

 another (Figs. 203, 209, pros). 

 204 When the cell-walls are thickened 



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

 sclerenchyma (Gr. scleros, 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 Rubber), 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 con tents, ei ther odoriferous, acrid, 

 colored, oily, or resinous, which "find 

 BO 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. 20.5. Prosenchyma of Indian Mallow (Aiu- 

 tilon Aziicennx). Fig. 2U0. Sclerenchyma from a Pear. 



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