10 



PEACTICAL BOTANY 



position, and a mass of distended cells pressing against one 

 another makes the whole structure rigid. But as when the air 



Fig. 7. A photograph of a Cottonwood leaf from which the green tissue has 



been removed so as to show the network of veins tlirough which the food 



material is carried throughout the leaf and to the stem 



Natural size. Leaf prepared hy EUswortli Hetlifl 



escapes from a balloon its wall collapses of its own \\'eight, 

 so do the cells of the leaves and shoots when, through loss 

 of water, they lose their turgidity. When soil water is not 



