LESSON 4.] STRUCTUKE OP VEGETABLE TISSUES. 



13 



Pith of Elder, porous cells. 



13), of which the accompanying figure of FIG. is. 



the pith of Elder affords a good example. 

 Authors have figured this tissue as if com- 

 posed of many sides (polyhedral), but this 

 appears to have arisen from the section being 

 too thick; the tissue is a singularly transpa- 

 rent one, and if three or four layers of cells 

 be present, the outlines of the cell walls run 

 together in such manner as to create great 

 confusion ; this is shown in Fig. 14 ; but if the section do not in- 

 clude more than two layers (Fig. 13), it will be seen that one layer 

 is simply aside of the other. 



74. But to return to Cells ; we have seen that the envelopes, or 

 outer walls of these, are composed of Cellulose ; generally, they are 

 found to possess contents of some kind. The Cellulose itself is 

 composed of sugar, gum, or vegetable jelly, which offers another form 

 of its chemistry, and while all plants are made up of similar cells, the 

 chief difference amongst them consists in the different form of the cells. 



75. It frequently happens that only a nucleus is found ; this 

 differs materially in size and color ; thus in the fruit of a ripe Cherry 

 (Fig. 15), we have a series of cells widely differing in size, each con- 



FIG. 14 



FIG. 15. 



Pith of Elder, according to authors. 



Transverse section of a Cherry. 



taining a large, fleshy, brownish-colored nucleus (a), the size of which 

 approximates to the dimensions of the cell containing it. Its surface 

 is seen to be covered with a series of minute spots, of greater inten- 

 sity of color the nucleoli, or smaller nuclei. 



76. Other cells are formed for the reception of special contents, 

 which, as will be hereafter seen, are extremely various. 



77. At the head of this list, however, fecula, or starch, in rela- 

 tion to human necessities, is most conspicuous; almost two-thirds 

 of the human family being nourished exclusively upon Starch, 



