156 MINUTE STRUCTURE OF THE LEAP. 



436. The growth of the j'oung leaf is plainlj' terminal at first, 

 — that is, new cells are added just in front of the older ones; but 

 it soon becomes intercalary as weU, new cells being introduced 

 between those previously existing. According to the seat of 

 activitj', this growth may be basipetal (the zone of growth being 

 near the base of the leaf-blade) or basifugal (the zone nearer 

 the apex of the leaf). In most cases the base of the leaf-blade 

 and the stipules early attain a good degree of development, after 

 which the petiole appears. 



For the purpose of noting the peculiar mode in which the leaf- 

 blade expands, the simple device suggested by Hales '■ is perhaps 

 as good as any. Through a piece of stiff pasteboard sharp pins 

 are thrust, and fastened at equal distances from each other ; for 

 instance, so as to form little squares of J inch side. By this sim- 

 ple instrument a young leaf is pierced through with holes at equal 



distances ; then if the leaf elon- 

 gates more than it widens in the 

 space thus covered, the holes will 

 separate in the direction of the 

 length of the leaf more than in that 

 of its width. The injury done to 

 the leaf by these small perforations 

 does not appear to check or other- 

 wise much modify its growth. 



437. Fibro-yascular bnndles. 

 The distribution of fibro-vascular 

 bundles in leaves has been con- 

 sidered in Vol. I., under "Vena- 

 tion." The two principal tj^pes of 

 distribution of the bundles, there 

 spoken of as "veins " or "nerves," 

 were shown to be (1) parallel, 

 (2) reticulated. Parallel venation 

 (see Fig. 119) is characterized by 

 having large "veins " or " nerves " 

 running free through the leaf (that 

 is, not connecting with each other), 

 or without any obvious anastomo- 

 sis ; while in reticulated venation 

 the veins form a more or less com- 

 plicated network. 



1 Statical Essays, vol. i., 1731, p. 344. 

 Fia. 119. Venationof theleaf ofOonvallarlalatifoUa. (Ettingshausen. ) 



