!6 On the Growth of Leaves. [Sess. 



Objects in Spirit, d-c. 



1. Cast skins of larvae, 1st type, AeschnidEe, 2 specimens. 



2. Larva, 2d n LibellnlidaB. 



3. ,, 3d m Agrionidss. 



4. Eggs. 



5. Dragon-fly, Agrion minium. 



6. it two species from foot of Ben. Lawers. 



7. Six foreign dragon-flies from Demerara. 



8. Six specimens British Aeschnina and Libellulina. 



II_0jV THE GROWTH OF LEAVES. 



By T. B. SPRAGUE, M.A., LL.D., &c, President. 



(Read Dec. 23, 1891.) 



To the student of natural history, who is also a mathematic- 

 ian, it is a matter of interest to investigate the exact laws 

 and processes according to which organisms grow. In some 

 cases where an organism grows for many years, we are able 

 to trace without difficulty the several stages of growth — as, 

 for instance, in the rings of a dicotyledonous tree. Instances 

 of a different kind, but illustrating the same principle, • are 

 found in land and sea shells of various kinds ; in each case 

 the new year's growth being added to the old shell in a way 

 which is easy to trace. In other cases the process of growth 

 is not so obvious ; for instance, the growth of the human 

 skull takes place by each of its component parts receiving 

 additions along the sutures. The bones of mammals grow 

 according to laws which are now well understood, and are 

 easily investigated in consequence of the discovery accidentally 

 made, that, when an animal has madder mixed with its food, 

 the new bone growth, which is ordinarily white, becomes a 

 pink colour ; so that, by giving an animal alternately food 

 that contains madder and food that is free from it, the bones 

 will contain pink and white layers alternately. 



As a contribution to the general subject, I propose to 

 mention to the Society an observation I have made this 



