358 HOW CROPS GROW. 
acid or alkali is carried beyond a point peculiar to each 
substance, contraction of the colloid takes place. The 
colloids just named acquire the power of combining with 
an increased proportion of water and: of forming higher 
gelatinous hydrates in consequence of contact with dilute 
acid or alkaline reagents. Even parchment-paper is more 
elongated in an alkaline solution than in pure water. 
‘When thus hydrated and dilated, the colloids present an 
extreme osmotic sensibility.” 
An illustration of membrane-diffusion which is highly 
instructive and easy to produce, is the following : 
A ae is scooped out in 2 carrot, as in fig. 68, so that 
x the sides remain } chi or so slick, and a 
quantity of dry, crushed sugar is introduced ; c 
after some time, the previously dry sugar will 
be converted into a syrup by withdrawing 
water from the flesh of the carrot. At the 
same time the latter will visibly shrink from 
the loss of a portion of its liquid contents. In 
this case the small portions of juice moistening 
the cavity form a strong solution with the 
sugar in contact with them, into which water diffuses from 
the adjoining cells. Doubtless, also, | sugar penetrates the 
parenchyma of the carrot. 
In the same manner, sugar, when sprinkled over thin- 
skinned fruits, shortly forms a syrup with the water which 
it thus withdraws from them, and salt packed with fresh 
meat runs to brine by the exosmose of the juices of the 
flesh. Jn these cases the fruit and the meat shrink as a 
result of the loss of water. 
Graham observed gum tragacanth, which is insoluble in 
water, to cause a rapid passage of water through a mem- 
brane in the same manner from its power of imbibition, 
. although here there could be no exosmose or outward 
movement. 
The application of these facts and principles to explain- 
Fig. 68. 
