240 PENNYP ACKER— ON THE BEACH PLUM 



■5hown as belonging to this variety, should not be considered as wholly 

 characteristic of it. They may occur on the reds and blues, even though 

 they are not shown on the photographs, but I have never found any on 

 the yellows. This purple type grew under intermediate conditions 

 as the branching would seem to indicate. The short lateral branches 

 — which usually fruit well — shown on Plate LXVI, Fig. 1 will later become 

 sub-spinescent and resemble the spine like branches of Plate LXVI, 

 Fig. 2. 



(b) Internal Structure, (microscopic): — The internal structure of 

 the stem which is shown on Plate LXVIII in cross section conforms to 

 previously described woody stems of the Rosaceae, of which the following 

 parts may be summarized briefly: — Epidermis rather thin, dark reddish 

 brown, which by the end of the first year is beginning to be shed. By 

 the end of the third or fourth year a thin bark has formed which sloughs 

 off in thin persistent scales or plates. Beneath is a layer of cork from 

 fifteen to thirty cells thick, which gradually pushes outward to form 

 the bark as new cork cells are cut off. These protective coverings sur- 

 round a rather thin cortex composed of ten to fifteen rows of parenchyma 

 cells; included in which are numerous clustered crystals of calcium 

 oxalate and numerous irregular masses of sclerenchymatous tissue or 

 stone cells. The latter are distributed around the stem in the inner 

 portion of the cortex. Beneath is a ring of rather loosely arranged 

 phloem tissue which extends outward from the wood in curious finger like 

 projections. Numerous air spaces are thus formed between them and the 

 rows of pith cells as they extend outward to join the cells of the medullary 

 rays with those of the cortex. The wood, which is composed of narrow 

 annular growths, is hard and compact. It contains numerous water 

 conducting vessels which vary in relative number in the different varie- 

 ties. It is also characterized by many broad medullary rays, widened- 

 out extensions of which separate the phloem into the finger like projec- 

 tions mentioned before as they pass outward to join the cortex cells. 

 The pith, which comprises the central portion of the stem, is solid and 

 compact, while the cells of it store starch during the resting period. 

 Clustered crystals of calcium oxalate are frequently but irregularly dis- 

 tributed throughout it. 



2. Leaf 



(a) External Structure: — The leaves opening with or after the blos- 

 soms, rarely before, vary in shape from oval to ovate to obovate. They 

 are rather firm in texture, greenish above paler beneath, and more pubes- 



