Morphology and Physiology of the Pome Lenticels of Pyrus malus

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Abstract
The pome lenticels, studied in 17 horticultural varieties of apples, were quite unlike the lenticels of woody stems. A lenticel arises from (1) a stoma, (2) a hypodermal break resulting from a fallen trichome, or (3) an epidermal break caused by stretching. The number of lenticels per pome is determined by the time the apple is 2-4 weeks old. The substomatal cells become modified by tannin and suberin and fail to develop the red or green pigments characteristic of the other hypodermal cells, and hence, the lenticel spots develop. Closing of lenticels is never associated with stomatal closure. The stomata remain open but the lenticels may close by (1) complete cutinization of the stoma, (2) formation of a cuticle over the exposed surface of the hypodermal cells, (3) complete suberization of the hypodermal cells, (4) development of a phellogen in the hypodermal cells. Closing is easily induced in young apples, but in fully matured apples or storage apples, closing is slow. Any condition of high temp. and low humidity favors closing. The number of lenticels per apple ranges from 300 in Winesaps to 2500 in Spitzenburger, with large variations within the variety. Neither water nor CO2 moves freely through the lenticel.

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