ELEMENTS OF ORGANIZED STRUCTURE. 253 



In one-celled plants, like yeast (Fig. 35), the new cells 

 thus formed, bud out from the side of the parent-cell, 

 and before they obtain full size become entirely detached 

 from it, or, as in higher plants, the new cells remain ad- 

 hering to the old, forming a tissue. 



In free cell-formation nuclei are observed to develop in 

 the protoplasm of a parent cell, which enlarge, surround 

 themselves with their own protoplasm and cell-membrane, 

 and by the resorption or death of the parent cell become 

 independent. 



The rapidity with which the vegetable cells may mul- 

 tiply 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 

 highest 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 

 imperf orate, they are nevertheless readily permeable to 

 liquids. This fact may be 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 pene- 

 trates 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 par- 



