.\o. 491] 



LEAF DEVELOPMENT 



709 



basally lobed proximal leaflets. This form appears with more 

 or less distinctness in Clematis virginiana, Rhus toxicodendron, 

 Rubm occidentalis, Ruhus strigosus, Negundo aceroides, Ailanthus 

 glandidosus, and Slum cicidacfoliam. By the predominance of 

 the basipetal or the basifugal element, palmate or pinnate leaves 

 are produced respectively. Twice pinnate leaves develop along 

 the same plan; in becoming twice pinnate a basipetal secondary 

 leaflet becomes matched by a smaller leaflet on the distal border 

 and further development of secondary leaflets in basifugal. This 

 is shown in' Sambucus, Aralia, and Cicuta, and the exceptional 

 nature of Bidens is recorded. 



The manner of leaf development in the rose requires further 

 study. The formation of leaflets in connection with stipules occurs 

 in Poterium, Sium, and the rose, but in Slum and to a less extent 

 in Poterium they form also from the terminal leaflet. T.obed 

 leaflets in the rose Avere not found. 



Jackson's studies have shown that some Cretaceous leaves are 

 like the simpler stages in the corresponding existing species, not- 

 ably in the tulip trees. Shull concludes, however, that "no 

 satisfactory inferences can be drawn from ontogenetic leaf-char- 

 acters regarding the phylogenetic history of the species." He 

 states that there is need of physiological interpretation, and further 

 biometric studies of leaves are being made. In connection with 

 paleontological and biometric studies it is important that the 

 embryology of leaves should be knowii, not by inference from 

 mature leaves but by reconstructions of the embryonic stages. 



The preceding descriptions of adult leaves show that there is 

 a determinate evolution of leaf forms, whereby diverse species tend 

 to produce similar shapes. Plants with simple leaves constantly 

 show tendencies toward compounding. The "obscurely lobed" 

 leaves of Malva rotundifolia are occasionally deeply divided, and 

 the notches on the red maple leaf may become clefts extruding to 

 the petiole. The persistent production of the similar torins of 

 compound leaves which have been described is evidence in favor 

 of determinate or orthogenetic evolution. 

 Cambridge, Mass. 



