26 THE ELEMENTARY STRUCTURE OF PLANTS. 



do not belong to the tissues themselves, but to the matters of differ- 

 ent colors which the cells contain. 



22. 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 is about the ^y or -^ of an inch. In the common Pink, 

 it has been computed that more than 5,000 cells are contained 

 in the space of half a cubic line, which is equivalent to almost 

 3,000,000 in a cubic inch. 



23. Cells are sometimes drawn out into tubes of a considerable 

 length, as in hairs, and the fibres of cotton, which are long and 

 attenuated cells. The hairs, or hair-like prolongations from the 

 surface of rootlets, are good examples of the kind. Two short 

 ones are seen in Fig. 1. In Fig. 13, 14, they are more fully 

 illustrated. 



24. Some idea may be formed respecting the rate of their pro- 

 duction, by comparing their average size in a given case with the 

 known amount of growth. Upon a fine day in the spring, many 

 stems shoot up at the rate of three or four inches in twenty-four 

 hours. When the Agave or Century-plant blooms in our conser- 

 vatories, its flower-stalk often grows at the rate of a foot a day ; it 

 is even said to grow with twice that rapidity in the sultry climes to 

 which it is indigenous. In such cases, new cells must be formed 

 at the rate of several millions a day. The rapid growth of Mush- 

 rooms has become proverbial. A gigantic Puff-ball has been 

 known to grow from an insignificant size to that of a large gourd 

 during a single night ; when the cells of which it is entirely com- 

 posed are computed to have been developed at the rate of three 

 or four hundred millions per hour. But this rapid increase in size 

 is owing, in great part, to the expansion of cells already formed. 



25. Development Of Cells, The whole potentiality of the plant 

 exists in the individual cells of which it is made up. In them its 

 products are elaborated, and all the vital operations carried on. 

 Growth consists in their production, multiplication, and enlarge- 

 ment. A knowledge of these processes is therefore requisite in 

 almost every inquiry that arises in physiological botany. Sys- 

 tematic botany and zoology, moreover, as well as anatomy and 

 physiology, both animal and vegetable, have advanced to the point 

 at which investigations into the development of organs are of the 



