386 HOW CROPS GKOIV. 



penetrated by the dye. It is therefore porous, not only 

 in the sense of having an interior cavity which becomes 

 visible by a high magnifying power, but likewise in hav- 

 ing throughout its apparently imperforate substance in- 

 numerable channels in which liquids can freely pass. 

 In like manner, all the vegetable tissues are more or less 

 penetrable to water. 



Imbibition of Liquids by Pbrous Bodies. — Not 

 oniv do the tissues of the plant admit of the access of 

 water into their pores, but they forcibly drink in or 

 aosoro tnis liquid, when it is presented to them in excess, 

 until their pores are full. 



When the molecules of a porous body have freedom 

 of motion, they separate from each other on imbibing a 

 liquid ; the body itself swells. Even powdered glass or 

 fine sand perceptibly increases in bulk by imbibing water. 

 Clay swells much more. Gelatinous silica, pectin, gum 

 tragacanth, and boiled starch hold a vastly greater amount 

 of water in their pores or among their molecules. 



In case of vegetable and animal tissues, or membranes, 

 we find a greater or less degree of expansibility from the 

 same cause, but here the structural connection of the 

 molecules puts a limit to their separation, and the result 

 of saturating them with a liquid is a state of turgidity 

 and tension, which subsides to one of yielding flabbiness 

 when the liquid is partially removed. 



The energy with which vegetable matters imbibe water 

 may be gathered from a well-known fact. In granite 

 quarries, long blocks of stone are split out by driving 

 plugs of dry wood into holes drilled along the desired 

 line of fracture and pouring water over the plugs. The 

 liquid penetrates the wood with immense force, and the 

 toughest rock is easily broken apart. 



The imbibing power of different tissues and vegetable 

 matters is widely diverse. In. general, the younger or- 

 gans or parts take up water most readily and freely. The 



