STRUCTURE.] DEVELOPMENT OF A LEAF. 283 



leaf, chiefly depend. The most essential of these differences 

 are as follows : 



1st. The most considerable development of a leaf at its 

 lower extremities. All measurements confirm this fact. I 

 have made some observations with this end in view, on 

 Polygonum hydropiper, the coloured spots on the leaves of 

 which serve very well as points of comparison. The follow- 

 ing are my results : 



1st leaf ; total length, 105 millim. ; from base to the spot, 58 millim. 

 2nd 98 50 



3rd 93 41 



4th 78 33 



5th 60 20 



6th 48 11 



7th 35 







The breadth of these seven leaves (which were taken from 

 a lateral branch) immediately below the spot was, for the 



1st leaf, 45 millim. 

 2nd 43 

 3rd 38 



4th , , 34 



5th leaf, 22 millim. 

 6th ,,15 

 7th ' 8 



Consequently, the increase in length of these leaves was 

 more considerable in their lower than in their upper half. In 

 the first three leaves, the most considerable breadth was at 

 the part measured, whilst in the other four the most consi- 

 derable breadth was above the spot. 



2nd. The generally more considerable development of the 

 lower leaflets of compound leaves, and the slower development 

 of the upper leaflets ; the lower leaflets are also often further 

 apart than the upper ones. 



3rd. At a certain period, stipules are generally larger 

 than the leaves or leaflets produced before them. It is only 

 in this case that stipules can be protecting organs. 



4th. The perishing of leaves from above downwards, (from 

 apex to base.) 



5th. The development of the petiole, which is analogous 

 to that of a blade of a leaf. The outer scales of leaf-buds, 

 which have been considered in many cases as leaves of a 



