56 



Science of Plant Life 



water balance in transplanted plants may prevent the loss of 

 many of them and may save weeks of delay in the maturing 

 of the crop. 



The water balance and plant habitats. The place where 

 a plant grows naturally is called its habitat. The willow grows 

 beside a stream and the cactus grows in the desert, each in its 

 natural habitat. If we put the willow in the desert and the 

 cactus on a wet stream bank, both die. This means that 

 the conditions that make up each habitat are favorable to 

 one kind of plant and not to another. The conditions in- 

 clude not only the kind of soil and the amount of soil water, 

 but also the evaporative power of the air. In selecting 

 plants that may Uve in a particular habitat, the great im- 

 portance of the dryness or the moistness of the air is to be kept 

 in mind. Plants whose leaves are soft and transpire water 

 rapidly can succeed only in moist air, while those that have a 

 low transpiration rate can maintain a suitable water balance 

 only in a dry atmosphere. This is one of the reasons why on 

 a southern slope we find a set of plants that are different from 

 those on the northern slope. 



Recent studies have shown that the 

 leaves of plants growing near the bottom 

 of a ravine transpire water lo to 20 

 times as fast as do those of plants 

 growing higher up on an adjoining 

 southern slope. Doubtless, each year 

 seeds of plants that grow in the low 

 ground germinate on the upper part of 

 the slope ; but each year the plants 

 that spring from those seeds are elimi- 

 nated through their inability to get the 



E. S. Clements 

 Fig. 36. Vertical sections of 

 leaves of Ilippiiris, a water 

 plant. The upper figure 

 shows an aerial leaf, the 

 lower figure a submerged 

 lekf. . 



