342 



SIB J. LUBBOCK PnTTOBIOLOGICAL OBSERYATIONB. 



leaves with strai<xlit ones, I su^i^ested that the first retains the 

 old form of leaf, while the latter has assumed one which presents 

 certain advantages. 





Fig. 1 . 



Leaf of Tamns, to show the 



Curved course of the veins. 



Leaf of Sycamore, to sliow the 



straidilTcourso of the veins. 



I also pointed out that in many cases the form of the leaf was 

 determined by the di^^tance between the buds taken in connection 



i 



with the strength of the stem, and illustrated this by numerous 

 examples. For instance, the Spanish Chestnut has a stronger 

 stem than the Beech, but the distances between the buds are not 

 much greater ; consequently the leaves may be much larger, but 

 cannot be much broader ; hence they assume the dagger-like form 

 characteristic of that species. Again, in Conifers, the species with 

 short leaves generally retain their leaves for several years, those 

 with longer ones for a shorter period ; the length of the leaf seeming 

 to be a function of the strength of the shoot, taken in relation to 

 the longevity of the leaf. The vast majority, indeed, of Conifers 

 are evergreens, though some few, such as the Larch (Larix), are 

 deciduous. This species flourishes especially in Siberia and the 

 higher alpine slopes of Switzerland, while elsewhere the^Spruce 

 gains the upper hand. These two species are deadly enemies. 

 Generally the Spruce has the advantage, as the young Spruce wdl 

 grow under Larch, while the shade of the Spruce is fatal to most 

 other plants.. But in Siberia and in the upper Alpine slopes of 

 Switzerland the short summer is perhaps too short for the Spruce, 

 while giving suflScient time for the Larch. 



It has been suggested by Mr. Grrant Allen that the tendency 

 to division in subaqueous leaves is due to the comparative paucity 

 of carbonic acid in water as compared with air. I have ventured 



to suggest a different explanation. Of course it is important to. 



