436 Journal of Agriculture. [10 July, 1909. 



But the stem and roots have contrivances for the admission of air and 

 other gases as well as the leaves, and since the first skin or epidermis 

 perishes and is replaced by a layer of cork, whenever they become too bulk\ 

 to be sufficiently protected by a single layer of cells, as in woody branches 

 of one year's growth, then they have a difl'erent arrangement. The corky 

 layer is easilv recognised in the Cork Oak, but it must be remembered that 

 this substance occurs generally in plants, although it would not be regarded 

 as such from the cork merchant's point of view ; as in the skin of the 

 potato. In many of our gum trees which are said to " shed their bark 

 instead of their leaves," this is simplv a shedding of the corky layer which 

 has become too small for the expanding stem, and if the smooth surface 

 beneath is scratched, even with the finger-nail, it will show the green 

 assimilating cells which are not found inside the proper bark, as we 

 know it, for instance, in the apple tree. 



2. CROSS SECTION OF LENTICEL ON SKIN OF POTATO (AFTER SORAUER). 



The structures found in those parts of plants invested b\ a corky 

 layer, corresponding in function to the stomata in the leaves, are called 

 Lenticels or Cortical pores. To the naked eye, they appear as little 

 M-arts on the surface of the stem, and just as the breathing-pores of the 

 leaves are formed from epidermal cells, so the breathing- pores of the stem 

 are formed from cork-cells. At these spots, the cork-cells do not form an 

 enclosed layer as they usually do, consisting of flattened cells arranged in 

 rows one above the other, tier upon tier, as shown in Fig. 2k. Where a 

 lentical occurs, the cork-cells are not firniK united together but loosely 

 arranged, leaving small spaces between which the air has free access to 

 the interior. 



In the cross-section of the skin of a pot.ito (Fig. :;) it is seen to be 

 composed of cork-cells, as at k, l>eneath which are the cells containing 

 starch-grains, and at a, a lenticel is beginning to be formed. The cells 

 take in moisture, swell, and cause the skin to burst, as at /. and the lenticel 

 thus formed is filled with loose whitish mealy cells. 



Even in normal growing plants, the wliite mealy appearance may occur 

 after a long spell of wet weather and I have seen it on the surface of 

 potatoes in wet growing seasons. Under the.se conditions, the cells at / 

 are multiplied excessively, they are pushe.l up from below, and the outer- 



