1920] DORSEY & WEISS— PETIOLAR GLANDS 401 



begins. Some of the species which show the separation of the old 

 leaf scar more clearly even than Amygdalus are Shepherdia argentea, 

 Cornus stolonifera, Tilia americana, Rhamnus cathartica, and Celtis 

 occidentalism the first two of which are illustrated in figs. 16 and 

 17. All of these have simple leaves and lack petiolar glands, and 

 Coxnus and Shepherdia are exstipulate and have entire leaves, 

 which, according to Sinnott and Bailey (ii, 12), is an advanced 

 state of node and leaf morphology. Hence we must regard the 

 structure to which the stipules and petiole in Prunus are articu- 

 lated, not as an additional foliar element, but as an outgrowth 

 from the stem; and therefore abscission of this kind does not 

 necessarily indicate an additional foliar element. 



The manner of insertion and abscission of the stipules in the 

 plum furnishes additional evidence that the structure described 

 by Cook as a persistent leaf base is in reality a part of the stem, 

 and that the leaves of Amygdalaceae cannot therefore be jointed. 



In some Rosaceous genera, as Potentilla and Rosa, the stipules 

 are adnate to the petiole, forming a somewhat sheathing base, and 

 fall with the leaf. In others, as Pyrus and Prunus, the stipules 

 are separate, or nearly separate, from the petiole, and absciss soon 

 after the buds unfold. In Prunus the stipules usually drop long 

 before the leaf, but occasionally they persist throughout the grow- 

 ing season, and even over winter, in vigorous, late growing branches 

 in which cold weather has stopped further growth. The point 

 of normal abscission is illustrated in fig. 32 (P. hortulana mineri). 

 The stipules, like the petiole, separate at a definite abscission 

 layer at their base (figs. 1,2). As Cook pointed out, the joint of 

 the latter lies above the stipules, that is, distally to them, although 

 since the abscission lines of the petiole and the stipules form a 

 sort of crescent with the points upward, a face view of the stem 

 shows the stipule scars above that of the petiole (figs. 31, 32). 



The typical leaves of many stipulate genera of the Rosaceae 

 have stipules adnate to the petiole, forming a more or less sheathing 

 base. This condition is also to be found in the bud scales and the 

 scales transitional to leaves in Prunus. Morphologists regard bud 

 scales as relatively primitive in structure, since they have not 

 specialized to serve such varied functions as the leaves themselves. 



