46 Principles of Plant Culture. 



plants to be constantly saturated with water. Indeed, it 

 distends the elastic cell-walls with water nntil they are 

 like minute inflated bladders. The pressure thus set up 

 aids in imfolding the different parts from their snug rest- 

 ing place within the seed-case and enables the plantlet 

 to stand erect. Growth by cell division, it is true, be- 

 gins rather early in the germination process, but this 

 cannot take place unless the cells are first distended with 

 water (29). A sufficient amount of water is absolutely 

 necessary, therefore, to growth in j)lants. Foliage wilts 

 in dry weather because the roots are unable to supply 

 enough water to properly distend the cells; growth is 

 impossible in plants of which the foliage is wilted. 

 When the water supply is abundant, on the other hand, 

 and the absorptive po^\er of the loots is stimulated by a 

 warm soil (102), the pressure within the cells often be- 

 comes sufficient to force water fiom the edges and tips 

 of leaves. The drops of water that so often sparkle on 

 foliage in the sunlight of summer mornings, commonly 

 mistaken for dew, are usually excreted from the leaves. 

 In young plants of the caladium, water is sometimes 

 ejected from the leaf-tii^s with considerable force. 



The -n-ater of plants is almost wholly absorbed by the 

 root-hairs (101), the leaves having no power to take irp 

 w^ater, e\en 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 bit of aniline blue into a glass of 

 clear water, it will not retain its form and size, but infi- 

 nitely small particles will become detached and move 



