188 



ELEMENTARY STUDIES IN BOTANY 



through the green tissue there extend " veins " (Fig. 154), as 

 described in 55 (p. 90). These veins are extensions of the 

 vascular system, and carry water to the green tissue. It is 

 \ necessary, therefore, that the veins branch sufficiently to 

 \reach all the working cells. How completely this is accom- 

 plished may be seen in a " skeletonized " leaf, from which 



^11 the green tissue has been removed, and only the veins 



C 



FIG. 154. A, pinnate leaf of quince, which is entire and has a short petiole; B, 

 palmate leaf of geranium, which is lobed and has a long petiole ; C, parallel-veined 

 leaf of lily-of-the-valley. After GRAY. 



remain (Fig. 155). Incidentally, but very necessarily, the 

 vein system forms a stiff framework to support the expanded 

 green tissue, which otherwise would collapse. That the veins 

 are not the only mechanical support is evident when a leaf 

 wilts, a thing which, as every one knows, is due to a lack of 

 water. For an ordinary foliage leaf to keep its expanded 

 form, the working cells must be gorged with water, and much 

 of the stiffness of a broad leaf is due to this turgor ( 10, 

 p. 10) of the cells. The green tissue, therefore, is kept spread 



