i [ [ 



I I I 



8 SOTANT. 



best microscopes. Cells which exist hy themselves, as in 

 many microscopic water plants, are more or less spherical; 

 so, too, are many spores and pollen-cells, and the cells of 

 many ripe fruits, where, in the process of ripening, the 

 cells have separated from each other. Ordinarily, how- 

 ever, the cells are of irregular shapes, on account of their 

 mutual pressure. Occasionally they are cubical, rarely 

 they are regular twelve-sided figures (dodecahedra), but 

 more commonly they are irregular polyhedra. 



12. In some of the lower aquatic plants cells occur 

 which for a time have no cell-wall (e.g. zoospores), but after 

 a short period of activity they come to rest and cover 

 themselves with a wall of cellulose. In some lower plants 

 also the cells contain more than one nucleus (e.g. in Water- 

 net, Water-flannel, etc.). In most plants, however, the 

 walled cells, each containing a single nucleus, are the units 

 of which the plant is composed, and in the study of differ- 

 ent plants, no matter how much they may differ in external 

 appearance, we shall always find that they are made up of 

 cells alike in all essential features. Thus the simple Green 

 Slime of the rocks is composed of a single cell, the homo- 

 logue of which is repeated millions of times in the giant 

 oak of the forests. 



. Practical Studies. — (a) Mount a leaf of a moss for a good example 

 of cells showing their walls. The sections of root-tips previously 

 mentioned (p. 5) may be studied again with profit. 



(6) For thickened cell-walls make sections of the shell of the 

 hickory-nut or cocoanut.] 



(c) Make longitudinal and also cross sections of apple-twigs ; some 

 of the pith-cells show thickened walls marked by dots and pits. 



{d) Make the following tests upon cell-walls : Apply sulphuric acid 

 and iodine — the cellulose-walls will turn blue or violet, the cutiu and 

 lignin walls yellow or brown. To separate the latter apply aniline- 

 water safranin, which stains the cutin-walls a yellowish and the 

 lignin-walls a bluish color. 



