54 VITAL ACTIONS. 



at the time wlien the leaves of other plants are perish- 

 ing ; and do not cast them till a new spring has com- 

 menced, when other trees are leafing, or even later. 

 In the latter class, the functions of the leaves are 

 going on during all the winter, although languidly ; 

 they are constantly attracting sap from the earth 

 through the spongelets, and are, therefore, in a state 

 of slow but continual winter growth. It usually hap- 

 pens that the perspiratory organs of these plants are 

 less active than in deciduous species. 



78. In general, a leaf is an organ of digestion and 

 respiration, and nothing more ; some leaves have, 

 however, the power of forming leaf-buds, if placed in 

 or upon earth, under suitable circumstances. The 

 Bryophyllum calcinum forms buds at the indenta- 

 tions of its margin ; Malaxis paludosa throws off 

 young buds from its margin ; Tellima grandiflora 

 occasionally .buds at the margins of its leaves: the 

 same thing happens to many Ferns ; and several 

 other cases are known. 



