1920] DORSEY & WEISS— PETIOLAR GLANDS 403 



jointed, led Cook to conclude that the ancestral leaf type of this 

 group was compound, the nectaries representing " rudiments of 

 divisions of compound leaves." It should be pointed out, however, 

 that if the modern plum leaf represents the terminal leaflet of an 

 originally compound leaf, the glands, representing reduced lateral 

 leaflets, should be found below the joint by which the terminal 

 leaflet is articulated to the rachis. It is obvious that being situated 

 on the petiole above the joint, they cannot represent lateral leaf- 

 lets of an ancestral pinnate form. Microscopic sections of the 

 petiole at the leaf base do not show an abscission layer subtending 

 the blade, and the presence of one is not indicated by the normal 

 manner of shedding the leaves. It may be safely concluded, there- 

 fore, that the leaf blade of the plum is not articulated to the 

 petiole, as in Citrus, Berberis, or Trifolium. Sometimes, however, 

 if there has been late growth in the fall and the leaves are immature 

 when the first frosts occur, the leaf axis may be broken anywhere 

 from the middle of the blade to the base of the petiole, although 

 most frequently at the juncture of the blade and petiole. In this 

 case the twig enters the winter with a part or all of the petiole 

 adhering at each node, which, however, usually breaks off at the 

 point of normal abscission by spring. 



Glands, however, may represent divisions of an alternate 

 divided or pinnatifid leaf, such as Fragaria, Potentilla, and other 

 genera of the Rosaceae possess. The frequency of the occurrence 

 of two glands suggests a ternate leaf such as that of Fragaria. 

 The presence of additional glands may be accounted for on the 

 basis of branching of the lateral bundles, or by the common transi- 

 tion from ternate to quinquefoliate or pinnate leaves (Lewis 

 7>8). Some pinnate-leaved forms of Potentilla show reduction of 

 the lower divisions to smaller structures than are the leafy out- 

 growths in the position of glands in the apricot. Furthermore, 

 the lower divisions are frequently alternate instead of paired, as 

 is also the case with the petiolar glands in Prunus. 



It will be recalled that in the plum the two lateral bundles 

 which connect with the glands terminate there, and do not con- 

 tribute extensively to the vascular system of the blade. In 

 ternate leaves which have three strands, the outer two provide the 



