200 Sir John Luhhoch [Feb 13, 



In many genera witli broad and narrow leaved species, Drosera and 

 Plantago, for instance, the broad leaves formed a horizontal rosette, 

 \vhile the narrow ones were raised upwards. Fleshy leaves were 

 principally foimd in hot and dry countries, where this peculiarity had 

 the advantage of offering a smaller surface, and therefore exposing 

 the plant less to the loss of water by evaporation. 



Water Plahts, 



Many of the acquatic plants have two kind of leaves — one more 

 or less rounded, which floats on the surface, and others cut up into 

 narrow filaments, which remain below; the latter thus present a 

 greater extent of surface. In air, however, such leaves would be 

 unable to support even their own weight, much less to resist any 

 force such as that of the wind. In perfectly still air, for the 

 same reason, finely divided leaves may be an advantage, while in 

 comparatively exjDOsed situations more compact leaves may be more 

 suitable. It was pointed out that finely cut leaves are common among 

 low herbs, and that some families which among the low and herb-like 

 species have such leaves, in shrubby or ligneous ones have leaves 

 more or less like those of the Laurel or Beech. 



An interesting part of the subject is connected with the light 

 thrown by the leaves of seedlings. Thus the Furze has at first 

 trifoliate leaves, which gradually pass into spines. This shows that 

 the Furze is descended from ancestors which had trifoliate leaves, as 

 so many of its congeners have now. Similarly in some species, 

 which when mature have palmate leaves, those of the seedling are 

 heart-shaped. Could it be possible that the palmate form was 

 derived from the heart-shaped, and that when in any genus we find 

 heart-shaped and lobed leaves, the former may represent the earlier 

 or ancestral condition ? He then pointed out that if there was some 

 definite form told off for each species then surely a similar rule ought 

 to hold good for each genus. The species of a genus might well 

 differ more from one another than the varieties of any particular 

 species ; the generic type might be, so to say, less closely limited ; 

 but still there ought to be some type characteristic of the genus. He 

 took then one genus, that of Senecio (the Groundsel). Now, in 

 addition to Senecios more or less resembling the common Groundsel, 

 there were species with leaves like the Daisy, bushy species with 

 leaves like the Privet and the Box, small trees with leaves like the 

 Laurel and- the Poplar, climbing species like the Tamus and Bryony. 

 In fact, the list is a very long one, and showed that there is no 

 definite type of leaf, but that the form in the various species depends 

 on the condition of the sj^ecies. From these and other considerations 

 he concluded that the form of leaves did not depend on any inherent 

 tendency, but on the structure and organisation, the habits and 

 requirements of the plant. Of course it might be that the present 



