

402 BOTANICAL GAZETTE [may 



Accordingly, the bud scales of Prunus may be taken as an index 

 of the ancestral leaf type. A series of bud scales and scale leaves 

 grading into true leaves is shown in figs. 18-29, which were taken 

 from a young shoot of the Compass cherry (P. BesseyiXP- hor- 

 tulana miner 1). 



It may clearly be seen that in the outer scales the stipules are 

 represented by blunt lobes, as large as the central lobe which 

 represents the leaf blade. The inner scales show progressive 

 reduction in size of the lateral lobes and increase of the middle 

 one, accompanied by differentiation into stipules and lamina. It 

 will be noted, also, that there is progressive splitting of the stipules 

 from the petiole. .In the mature leaf of the plum this splitting 

 has progressed to the base of the petiole, so that leaf and stipules 

 have separate abscission, but from one originally continuous 

 abscission layer. Prunus avium, as represented by the Dyehouse 

 cherry, is at a somewhat intermediate stage. In the leaves near 

 the base of a shoot, the stipules are clearly adherent to the petiole, 

 while in the upper leaves they are separate, as in the plum. In 

 all these transitional bud scales separation from the axis is clearly 

 below the stipules, and there is no evidence of an abscission layer 

 above them cutting off the middle lobe or leaf blade (fig. 2) ; hence 

 the stipules in the plum must be regarded morphologically as 

 integral with the leaf base, although the course of development 



in the mature 



in 



relation to a more primitive type of leaf. Their organization and 

 their occasional proliferation into leaflike outgrowths indicate that 

 they are reduced structures. This change from a glandular to a 

 foliar structure was noted by Cook (i), who described the occur- 

 rence of "small oblong or spatulate leafy organs on the upper 

 part of the petiole, taking the place of the nectaries" in certain 

 varieties of apricots. A similar transformation has been observed 

 in Crataegus, as well as a number of species of Prunus, and in some 

 of the apricot hybrids at the Minnesota Experiment Station it is 

 of almost regular occurrence on vigorous shoots of young trees. 

 Some of the variations in this transformation are illustrated in 

 figs. n-15. The view that the leaves of the Amygdalaceae are 



