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. 
§ 2, 
. 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 haye been distinguished by vegetable anat- 
