242 



PENNYPACKER— ON THE BEACH PLUM 



VARIETY 



Large Blue 



Small Blue (1). 



(2). 



Large Purple... 



Small Purple 



Large Red 



Small Red 



Large Yellow... 

 Small Yellow... 



UPPER EPIDERMIS 



Pubescent along mid rib and lead- 

 ing veins 

 Pubescent only along midrib 



Pubescent along midrib, veins and 



cross veins 



Pubescent only along midrib 



Pubescent only along midrib 



Pubescent only along midrib 



Pubescent only along midrib 



Pubescent along midrib slightly 

 along the leading veins 

 Pubescent over whole surface, 

 midrib, veins and cross veins 



LOWER EPIDERMIS 



Pubescent only along 

 midrib 



Pubescent only along 

 midrib 



Very pubescent as des- 

 cribed for up. epid. 

 Pubescent only along 

 midrib 



Pubescent only along 

 midrib 



Pubescent only along 

 midrib 



Pubescent only along 

 midrib 



Pubescent only along 

 midrib 



Pubescent along mid- 

 rib, veins and cross 

 veins 



are sharply prominent beneath, where 6 to 8 veins usually occur on each 

 side of the midrib. 



(b) Internal Structure (microscopic). The leaf is characterized by 

 a thick epidermis that prevents too rapid transpiration of water. The 

 cells of the upper epidermis are 2 to 2 3^ times larger and more dense 

 than those of the lower. Frequently mucilage is deposited in the cells 

 of the upper, less often in the cells of the lower. Two rows of palisade 

 cells can usually be distinguished, but these are sometimes small and 

 hard to differentiate. The mesophyll is composed of an undifferentiated 

 mass of minute cells, which is very characteristic of all varieties, the 

 air spaces are numerous, but small, while along the midrib and main 

 veins are numerous clustered crystals of calcium oxalate. These vary 

 in size and relative number in the different varieties. In cross section, 

 the crystals are seen to be scattered throughout the mesophyll, some- 

 times in the palisade layers near the upper epidermis, at other times 

 in the undifferentiated mesophyll near the lower epidermis, but never 

 so far as I have been able to discover, are they located in the epidermis. 

 Frequently, these crystals vary in size in the same leaf and may be classi- 

 fied as large, medium, and small. The midrib contains a deeply crescen- 

 tic vascular bundle that occupies one-half of its area and is surrounded 

 by a loose cortex of mesophyll cells, which, like the leaf mesophyll, con- 



