232 HOW CROPS GROW. 



The rapidity with which the vegetable cells may multi- 

 ply and grow is illustrated by many familiar facts. The 

 most striking cases of quick growth are met with in the 

 mushroom family. Many will recollect having seen on the 

 morning of a June day, huge puff-balls, some as large as a 

 peck measure, on the surface of a moist meadow, where 

 the day before nothing of the kind was noticed. In such 

 sudden growth it has been estimated that the cells are 

 produced at the rate of three or four hundred millions per 

 hour. 



Permeability of Cells to Liquids. Although the high- 

 est magnifying power that can be brought to bear upon 

 the membranes of the vegetable cell fails to reveal any 

 apertures in them, they being, so far as the best-assisted 

 vision is concerned, completely continuous and imperforate, 

 they are nevertheless readily permeable to liquids. 

 This fact may be elegantly shown by placing a delicate 

 slice from a potato-tuber, immersed in water, under the 

 microscope, and then bringing a drop of solution of iodine 

 in contact with it. Instantly this reagent penetrates the 

 walls of the unbroken cells without perceptibly affecting 

 their appearance, and being absorbed by the starch-grains, 

 at once colors them intensely purplish- blue. The particles 

 of which the cell-walls and their contents are composed, 

 must be separated from each other by distances greater 

 than the diameter of the particles of water or of other 

 liquid matters which thus permeate the cells. 



THE VEGETABLE TISSUES. 



As already stated, the cells of the higher kinds of plants 

 are united together more or less firmly, and thus consti- 

 tute what are known as VEGETABLE TISSUES. Of these, 

 a large number have been distinguished by vegetable anat- 



