Modified Parts. 



141 



We can only conjecture how such modifications of form 

 and changes of function have come about. In the case of 

 cacti, we can see that the reduction of the leaves and the 

 thickening of the stems 

 have fitted them to in- 

 habit desert regions by a 

 reduction of the tran- 

 spiring surface, and an 

 increase of the water- 

 storage tissues, of which 

 the bulk of the plant con- 

 sists. In the case of 

 RiiscHs, it may be that 

 some time back in its 

 ancestry it became de- 

 sirable to reduce tran- 

 spiration on account of 

 scarcity of water, or in- 

 ability to absorb the water 

 because of its saltiness, 

 or because of the low 

 temperature of the soil 

 (see the Chapter on Ad- 

 aptation to environment); 



under which conditions the plants with the smallest leaves 

 might have fared the best and produced the greatest num- 

 ber of offspring. . Then it might have come about in the 

 course of time that the species was represented by indi- 

 viduals whose leaves were mere scales. Later, the condi- 

 tions might have changed so that water could be more 

 readily obtained ; it would then have been desirable to 

 increase surfaces for photosynthesis, but the scale leaves, 

 being poorly nourished and reduced members, would have 



Fig. 70. 



Shoot of Ruscus kypoglossum, showing leaf- 

 like stems or cladophylls. After Kerner. 



