44 Principles of Plant Culture. 
is largely due to a strong absorptive power for water posses- 
sed by the protoplasm within the cells. This force causes 
all living parts of plants to be constantly saturated with 
water. More than this, it distends the elastic cell-walls 
with water until they are like minute inflated bladders. The 
pressure thus set up aids in unfolding the different parts 
from their snug resting place within the seed-case, and en- 
ables the plantlet to stand erect. Growth by cell division, 
it is true, begins rather early in the germination process, 
but this cannot take place unless the cells are first distended 
with water. A sufficient amount of water is absolutely 
necessary, therefore, to growth in plants. Foliage wilts in 
dry weather because the roots are unable to supply enough 
water to properly distend the cells, but growth is impossible 
in plants of which the foliage is wilted. When the water 
supply is abundant, on the other hand, and the absorptive 
power of the roots is stimulated by a warm soil (102), the 
pressure within the cells often becomes sufficient to force 
water from the edges and tips of leaves. The drops of 
water that so often sparkle on foliage in the sunlight of 
summer mornings, commonly inistaken for dew, are usually 
excreted from the leaves. In young plants of the caladium, 
water is sometimes ejected from the leaf-tips with consider- 
able force. 
The water of plants is almost wholly absorbed by the 
root-hairs (101), the leaves having no power to take up 
water, even in wet weather. The water of plants, with its 
dissolved constituents, is commonly called sap, except in 
fruits, where it is usually called juice. 
64. How Food Materials are Distributed through the 
plant, If we drop a very small lump of aniline blue into 
