iQ2o] DORSEY & WEISS— PETIOLAR GLANDS 397 



was zero. This is significant in view of the fact that these three 

 forms, P. americana, P. domestica (Shippers' pride), and P. domes- 

 ticaXP. americana, showed the greatest suppression of the glands 

 of any of the forms included in this investigation. This condition 

 will be given even greater emphasis in view of the connection found 

 to exist between glands and the vascular system of the leaf. 



Connection of petiolar glands with vascular system 



With the status of the glands as to number and position shown 

 by the statistical analysis in mind, it now remains to be seen 

 whether or not there is a basic cause for the predominance of the 

 2-0 frequency. 



The leaf trace in the plum has three bundles of conducting 

 tissue at its departure from the vascular cylinder of the stem, 

 which cause three gaps in the woody cylinder. The central bundle 

 extends up the petiole, through the blade, and branches succes- 

 sively at the large lateral veins. The outer bundles give rise to 

 strands which run along the upper side of the petiole, forming 

 pronounced ridges on either side. These strands run directly to the 

 petiolar glands, or to the large ones borne on the leaf base, and 

 terminate there. The term petiolar gland, therefore, will be used 

 in this connection to include both. The lateral strands are shown 

 in fig. 30, which was drawn from a young leaf, and they are equally 

 conspicuous whether the glands are borne on the leaf base or some 

 distance down on the petiole. Where there is more than one 

 gland on either side of the petiole, branches from the lateral strands 

 connect with them. 



On the other hand, the glands of the leaf serrations have a 

 distinctly different vascular connection. Instead of being con- 

 nected by branches with the lateral strands, glands in this position 

 have their vascular connections with the central bundle through 

 the lateral veins. It appears, therefore, that on the basis of 

 differences in their vascular connections the petiolar glands and 

 the glands borne on the serrations can be placed in two distinct 

 classes. Other considerations also support this view. The petiolar 

 glands are much larger than those on the serrations and may 

 show differences in shape, such as the reniform or necked types, 



