CELLULAK TISSUE. 



25 



Fig. 3), the meshes consequently appear six-f' ,fd. If the organ 

 is growing in one direction more tlian another, the cells commonly 

 lengthen more or less in that 

 direction. It is not necessary to 

 detach a cell in order to ascertain 

 its shape ; that may usually be 

 inferred from the outlines of the 

 section in two or three directions. 



21. The shape of cells, there- 

 fore, when they compose a tissue, 

 depends very much upon the way 

 in which they ai-e arranged and 

 press upon each other. Wlien 

 separate, as they are in the sim- 

 plest vegetables, or when nearly 

 free from each other, like the hairs on the surface of many plants, 

 they determine their own form by their mode of growth, and assume 

 a great variety of shapes, some of which are shown in the follow- 

 ing illustrations. The natural and primitive foim may be said to 

 be roundish or spherical. By increased growth in one direction 

 they become oblong or cylindrical, or, when still more extended, 

 they become tubes. Of this kind are the hair-like prolongations 

 on the surface of young rootlets (shown just beginning in Fig. 1, 

 and more elongated in Fig. 135-137); and the fibres of cotton 

 are slender hairs, consisting of single, very long cells, growing on 

 the surface of the seed. 



22. The walls of young cells are transparent and colorless. The 

 various colors which the parts of the plant present, the green of^ 

 the foliage, and the vivid hues of the corolla, do not belong to the 

 tissues themselves, but to the matters of different colors which the 

 cells contain (92). As they become older, the walls often lose most 

 of their transparency, and even acquire peculiar colors, as in the 

 heai-t-wood of various trees. 



23. The cells vary greatly in size, not only in different plants, 

 but in different parts of the same plant. The largest are found in^ 

 aquatics, and in such plants as the Gourd, where some of them are 

 as much as one thirtieth of an inch in diameter. Their ordinary 

 diameter in vegetable tissue is between ^l^ and ^jtyw of an inch. 



FIG. 9- A small portion of the tissue of pith, seen hoth in transverse and longitudinal 

 section, much magnified. 



3 



