ELEMENTS OF ORGANIZED STRUCTURE. 247 



across a young cabbage-stalk. It exhibits the outline of 

 the irregular empty cells, the walls of which are, for the 

 most part, externally united and appear as one, a. At 

 the points indicated by 5, air-filled cavities between the 

 cells are seen, called intercellular spaces. A slice across 

 the potato-tuber (see Fig, 52, p. 300) has a similar ap- 

 pearance, except that the cells are filled with starch, and 



it would be scarcely pos- 

 sible to dissect them apart; 

 but when a potato is boiled 

 the starch - grains swell, 

 and the cells, in conse- 

 'quence, separate from each 

 other, a practical result of 

 which is to make the po- 

 tato mealy. A thin slice 

 of vegetable ivory (the seed 

 ofPhytelephas macro- 

 Fig, so. carpa) under the micro- 

 scope, dry or moistened with water, presents no evident 

 trace of cell-structure ; however, upon soaking in sul- 

 phuric acid, the mass softens and swells, and the indi- 

 vidual cells are revealed, their surfaces separating in 

 six-sided outlines. 



Form of Cells. In the soft, succulent parts of 

 plants, the cells lie loosely together, often with consider- 

 able intercellular spaces, and have mostly a rounded out- 

 line. In denser tissues, the cells are crowded together 

 in the least possible space, and hence often appear six- 

 sided when seen in cross-section, or twelve-sided if viewed 

 entire. A piece of honey-comb is an excellent illustra- 

 tion of the appearance of many forms of vegetable cell- 

 tissue. 



The pulp of an orange is the most evident example of 

 cell-tissue. The individual cells of the ripe orange may 

 be easily separated from each other. Being mature and 



