270 CAUSES OF DEGENERATIVE EVOLUTION 



the tissues of the crab, its digestive canal has been 

 lost. 



(3) Atrophy of the leaf. In many plants the 

 leaves have disappeared, their function having 

 been assumed by some other part of the plant, 

 as, for instance, by the phyllodes in Acacia and 

 in Phyllanthus (fig. 84). 



(4) Atrophy of the protonema in mosses, and of 

 the leaves in some xerophilous plants. At germina- 

 tion, mosses produce a much branched filamentous 

 structure which serves as the organ of nutrition, 

 and is termed the protonema. Later on this gives 

 rise to buds which develop into the normal leafy 

 shoots of the plants. As soon as the leaves are 

 large enough to manufacture food for the plant, the 

 protonema begins to degenerate, and disappears 

 completely, except in a few rare forms (Ephe- 

 meracese) where the leafy shoots remain very 

 small. 



In Muehlenleckia platy dados (fig. 80), which has 

 become adapted to arid regions, and in consequence 

 has the surface from which evaporation may take 

 place much reduced, the branches which do not 

 bear assimilating leaves are rounded at their bases, 

 but higher up flatten into broad blades. These 

 blades contain chlorophyll and fulfil the functions 

 of leaves, these latter being present only as minute 

 scales. 



Similar phenomena occur in many Papilion- 

 aceous plants belonging to the genera Genista 



