ECOLOGICAL GEOUPS 



483 



In order to realize the extreme danger to which plants are 

 exposed from dryness, one needs only to remember how often 

 harvests in great part fail from the effects of drought. This 

 may mean that the entire plants have been killed, or only that 

 they have not borne much fruit or seed, or that the roots, stems, 

 or leaves are stunted. Many wild plants are as sensitive to 

 prolonged drought as are ordinary field crops, and irrigation 

 of a desert region which has a 

 rich soil helps the growth of 

 weeds as much as it does that 

 of the crops among which they 

 spring up. 



444. Means of limiting 

 transpiration. Some of the 

 principal means of limiting 

 transpiration are as follows 1 : 



(1) Compact arrangement 

 of the parenchyma cells in the 

 interior of the leaf. 



(2) Development of a 

 thick-walled epidermis (Figs. 

 249 and 364). 



FIG. 364. Waterproof epidermis and 

 protected stoma of the century plant 



c, cuticle ; cu, cutinized (waterproofed) 

 layer of epidermis ; ce, cellulose layer 

 of epidermis ; pi, pit, at the bottom of 

 which the stoma is situated ; po, pore 

 of the stoma. Magnified about 220 

 diameters. After Luerssen 



(3) Situation of the sto- 

 mata in pits or furrows (Figs. 

 249 and 364). 



(4) Inclosing the stomata 

 in a sort of tubular cavity 



formed by the curving-in of the margins of the leaf (Fig. 363). 



(5) Presence of a coating of dead hairs, filled with air, on 

 one or both surfaces of the leaf (Fig. 57). 



(6) Temporary reduction of the evaporating surface, as 

 by rolling up leaves (Fig. 2), shedding leaves, reduction of 

 living parts to a buried root, bulb, tuber, rootstock, or some 

 combination of thickened roots and underground stems. 



1 The subject is a very extensive one, fully treated in the writings of 

 Warming, Schimper, Goebel, Volkens, and other Geologists, 



