196 A TEXTBOOK OF BOTANY [Cn. IV, 12 



ribs, which have the effect of increasing the spread of green 

 surface without a proportional increase of transpiration, 

 which, of course, is the ever-present danger to plants of dry 

 places (page 69). These ribs vary much in number, from 

 many to few, and even in some cases to two, when the struc- 

 ture approximates closely in appearance and function to a 

 single leaf (Fig. 142). Thus is presented still another exam- 

 ple of the attainment of the same functional end by a dif- 

 ferent morphological route. 



The explanation of such remarkable morphological-physi- 

 ological overturnings as are presented by the cladophylla 

 is probably to be found, as with similar anomalies in leaves, 

 in a devious course of evolution through conditions and 

 habits very different from those now distinctive of these 

 plants. 



12. THE MONSTROSITIES OF STEMS AND LEAVES 



It often happens that individual parts of plants grow so 

 differently from their usual method as to attract attention 

 and be designated " freaks." Scientifically such cases are 

 called ABNORMALITIES, or if extreme, MONSTROSITIES. Aside 

 from their interest as curious things needing explanation, 

 they are scientifically important for the light they throw 

 upon the methods of plant development. 



First, it must be noted that not all peculiar growths are 

 properly monstrosities, for many result from purely mechani- 

 cal causes. Thus, when a stem is encircled by a rigid ring 

 (e.g. supporting iron band or wire attachment of a label), 

 it becomes thereby constricted in its further growth, and 

 swells greatly above the obstruction, because of the ac- 

 cumulation of food stopped in its downward passage through 

 the bark (Fig. 107). Precisely this cause produces great 

 spiral ridges on trunks gripped by twining vines. Again, 

 different parts of the same plant often become grown or 

 grafted together, because crushed or rubbed against one an- 

 other when young. In this way twin fruits are sometimes 



