A STUDY IN PLANT VARIATION 239 



II. Structure 



1. Stem 



(a) External Structure: — The Beach Plum is a bush which usually 

 grows to a height of two to six feet and may then be from ten to twenty- 

 five years old. Occasionally plants from twelve to fifteen feet high 

 are found which indicate an age of eighty to one hundred years; the 

 main stem or trunk then measuring three to three and one-half inches 

 in diameter, at the level of the ground. The bark of the old stems is 

 rough, fissured, dark brown to gray, and usually covered with a per- 

 sistent growth of lichens. The younger branches are dark purpHsh 

 gray with numerous irregular, flattened lenticels. On the youngest 

 twigs the lenticels are more circular in outline and elevated. These 

 twigs are usually of a light silvery gray color, due to an epidermis which 

 is shed rather irregularly in the different varieties. After the shedding 

 of the epidermis, which is usually completed by the beginning of the 

 second year, the twigs are of a reddish brown color, the light gray len- 

 ticels being quite conspicuous. The epidermis, during the first year's 

 growth, is slightly puberulous. The leaf scars are small and crescent 

 shaped, and contain three small fibro-vascular bundle scars, which may 

 be seen only by aid of a hand lens. 



Branching: — The method of branching is quite marked and varies 

 more with the situation in which the bush may be growing than with 

 the variety. As far as I have been able to ascertain, the branching 

 should not be considered a constant factor, although it may apparently 

 seem to be from some of the plates incorporated in this thesis. The bushes 

 exposed to the cold sea breezes, and the sand laden winds are the most 

 stunted in their growth, the yearly additions being rarely more than 

 two or three inches. As a result, the stems are more sturdy and more 

 branched, like that shown on Plate LXVI, Fig. 1. This is quite 

 typical of the large blue variety^ and was taken from a bush bearing 

 that color of fruit. The yellow variety Plate LXVI, Fig. 4 illustrates the 

 other extreme of the series of photographs taken. This branch was taken 

 from a bush growing five-hundred yards from the shore where the more 

 sheltered conditions favored an elongated less branched type. However, 

 when the yellow variety grows nearer the ocean it more closely resembles 

 one of the red variety of which Plate LXVI, Fig. 3, is very typical. 

 The purple variety, a specimen of which is shown on Plate LXVI, Fig. 

 2, shows spine like lateral branches, which although they are only 



