H)gO AGRICULTURAL HOTANY, CliKMISTRY AND PHYSIOLOGY OF PLANTS 



848 - The Taxonomic Value and Structure of the Peach Leal Glands. - ciREcoRY c. 



T., in Cornell University Agricultural Experiment Station of I lie Xetc York State Collese 

 0/ .-li^rjc/tZ/ior, Bulletin 365, pp. 183-222 + plates. Ithaca, N. Y. , November 1915. 



Since the early part of the nineteenth century, the leaf glands have 

 been used by all systematists in the classification of peaches and necta- 

 rines. Recently some doubt has arisen as to the advisability of using them 

 for classificatory purposes because of their variability. 



In a large number of cases the glands are stable and can be safely used 

 to aid in the identification of certain varieties. There are also varieties 

 in which the glands are exceptionally unstable, being on the border line 

 between the two types- — reniform and globose — and having what might be 

 termed mixed glands. These mixed glands are of two kinds : one in which 

 the majority of the glands are reniform, intermingled with globose forms ; 

 the other in which the globose form predominates. It would be quite pos- 

 sible, as Carrie;re (1867) suggests, to distinguish a third type of gland — 

 the mixed type. It is important that leaves should be chosen from healthy 

 branches on bearing trees. It is also best to obtain a large number of leaves 

 or to examine the tree carefully before making the final selection of leaves. 

 Mature leaves are best because their glands are full-sized and correctly 

 shaped, while on young leaves the form of the glands is usually vague ; 

 old, partly decayed, globose glands frequently have much the appearance 

 of reniform glands. 



The structure of the glands shows that they are true glands, having 

 an upper layer of long, rectangular, secretory cells that produce a sweet 

 substance, the fimction of which is not apparent. After the glands have 

 ceased secreting they begin to decay, becoming brown on the upper sur- 

 face and slowly disappearing until almost nothing is left. This decaying is 

 a vory complicated process, being preceded in every case by a suberization 

 and thickening of the cell walls. The spines of the leaf are very similar to the 

 glands in structure, having the same upper layer of long cells, but with much 

 more heavily cutinised walls. A study of the transitional forms indicates 

 that the glands are merely modified leaf spines. 



The leaves with reniform glands are apparently the highest t^'pe and 

 the glaudless leaves the lowest, with the transition through the globose 



type. 



The serrations of the glandless leaves are very strikingly different from 

 those on a leaf with glands. The former leaves are deeply and doubly 

 serrate, while the margins of the latter are always single and crenate. Al- 

 most invariably, when glands develop on a normally glandless leaf, the 

 serrations are transformed to crenations, indicating that there is a very 

 close correlation between the glands and the crenations on the edges of the 

 leaves. 



In the appendix a list of 29 works on the subject is given. 



