226 



HOW CROPS GROW. 



Fig. 30. 



regular empty cells, the walls of which are, for the most 

 part, externally united and appear as one, a. At the points 

 indicated by #, cavities between the cells are seen, called 

 intercellular spaces. A slice across the potato-tuber, (see 

 fig. 52, p. 277,) has a similar appearance, except that the 



cells are filled with starch, 

 and it would be scarcely 

 possible to dissect them 

 apart ; but when a pota 

 to is boiled, the starch- 

 grains swell, and the cells, 

 in consequence, separate 

 from each other, a practi 

 cal result of which is to 

 make the potato mealy. 

 A thin slice of vegetable 

 ivory (the seed of Phy- 

 telephas macrocarpa)^ 

 under the microscope, dry or moistened with water, pre 

 sents no trace of cell-structure, the cells being united as 

 one ; however, upon soaking in sulphuric acid, the mass 

 softens and swells, and the individual cells are at once 

 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 considerable inter 

 cellular spaces, and have mostly a rounded outline. 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 illustration of the appear 

 ance 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 

 IK- &amp;lt;-:isily separated from each other, as they are one-fourth 

 of an inch or more in length. !&amp;gt;&amp;lt; ing mature and incapa 

 ble of further growth, they possess neither protoplasm nor 



