DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM. 



iSi 



On an originally monopodial branching depends, on the other hand, the form of 

 the pinnate, lobed, divided, and toothed foliage-leaves of Angiosperms^. The leaf 

 appears on the cone of growth as a roundish protuberance which quickly broadens into 

 a shell-like form (Fig. 140, ^, b), and grows vigorously at its apex. Beneath the apex 

 protuberances arise at the right and left in acropetal order; these also grow in 

 the same manner at their apex (/), and produce again lateral protuberances of the 

 second order (0) ; which, according to the extent to which the surface of the leaf is 

 developed, become lobes of a simple leaf or distinctly separated leaflets. 



When two rows of lateral branches arise successively on the median axis of the leaf, 

 they generally remain weaker than it, and their lateral branches are also less numerous 

 and weaker ; the development of such an originally monopodial branch-system of kaves 



Fig. 140.— Development of the pinnate leaves of 

 Unibelliferse ; A, B oi Pastinaca sativit; C of LcVi- 

 stictint officinale; A apical region of the primary 

 stem ; its cone of growth is seen at j, its youngest 

 leaf at b; b' youngest leaf but one With the pinnation 

 commencing; C. bs apex of tlie leaf ; y, y"', /" ' leaf- 

 branches of the first order ; ^ of the second order. 



FIG. 141. —Leaves of Amorphophallu bulbosUs ; A with a simple, B with 

 a tlireefold branching of the lamina;. 



is therefore racemose. But the development may also be cymose, atid rii^y even- lead to 

 the formation of sympodia, especially when only one branch arises right and left on the 

 primary leaf. This is the case, for instance, in the leaves of Helleborus^ Rubusy and of 

 several Aroideae, as Sauromaium and Amorphophallus . Fig. 141, ^ represents a weakly 

 leaf of the last-named plant with only one branch on each side ; but when the leaves 

 attain a more vigorous development, as shown at jB, each lateral lobe, 2 2, forms on its 

 outer side again a lobe of the third order, 3 3, which again produces a similar one of the 

 fourth order, 4 4, and so on. According to the general definitions given above, the first 

 branch of the leaf, i, forms with 22a dichasium; but each branch of the dichasium 



* This was first shown in detail by Nageli (Pflanzenphys. Untersuch. von Nageli und Cramer, 

 Heft 11) in the leaves of Aralia spinosa, — See Eichler, Zur Entwickelungsgeschichte des Blattes 

 (Dissertation, Marburg, 1861). 



