282 DEVELOPMENT OF A LEAF. [BOOK i. 



what are owing to a growth which takes place in all directions. 

 A considerable increase of size is the consequence, but it is 

 not likely that this increase affects the mass in the same 

 degree. The form of the leaf suffers considerable altera- 

 tions. These changes, however, proceed in a much more 

 rapid and uniform manner, in all the parts, than the preceding 

 developments. 



Measurements made on the increase of leaves have fur- 

 nished results, which though curious can have no real value, 

 so long as the causes of this phenomenon, and the way in 

 which it takes place, are unknown. 



The development of a leaf always begins at the oldest 

 part of the rudimentary leaf, viz., at its apex, whence it 

 proceeds towards its base. The development of all folia- 

 ceous organs appears to proceed in the same way in this 

 respect; but the rapidity with which it proceeds, and the 

 size to which it extends, vary considerably according to the 

 situations of these organs. It is, in general, only in conse- 

 quence of this development that the blade, the petiole, and 

 other constituent parts of a leaf are formed, and consequently 

 these organs could not have existed from the very first. 



When a leaf, or certain parts of a leaf, become distinct 

 from the axis, their immediate connection with the latter is 

 more or less destroyed, and their respective position is 

 changed. This takes place more especially with the apex of 

 the leaf, which is placed the furthest from the axis. The 

 leaf, nevertheless, still continues dependent on the axis, 

 especially as concerns those parts which are the closest to it. 

 Now, as it is always the base of a leaf that is in this con- 

 dition, we must refer the force and rapidity of its development 

 to this cause. 



Anatomy shows us that the vascular bundles of the axis 

 penetrate into the base of a leaf before they arrive at its 

 apex. Moreover, liquids absorbed by the roots, penetrating 

 the axis and lower leaves, arrive at the base sooner than at 

 the summit of a leaf, where their constituent parts have 

 already undergone considerable modifications. 



It is on these conditions that the differences observed 

 between the development of the lower and upper part of a 



