106 THE AMERICAN MONTHLY [May, 



develops, and its leaves furnish the food for a growth which is 

 preparatory to the maturation of the seeds. The potato tuber 

 shows a great uniformity of histological structure, there being 

 many thousands of starch-filled pith-cells for every cell of any 

 other kind, and it presents a case of the application of division of 

 labor and the production of a large aggregate tolerably uniform 

 in character. It will be well to begin a description with the epi- 

 dermis or bark. This is to be studied by thin sections, cut par- 

 allel to the surface, supplemented by vertical sections. Figure 2 

 is a drawing, magnified about ten times, of a slice vertical to the 

 surface and passing through a bud. The thin outer band of this 

 section is the hypodermis or bark. The band parallel with this 

 passing up into the bud is the bundle of fibro-vascular tissue which 

 corresponds with the wood of stout'woody stems, while the re- 

 mainder is pith or parenchyma cells, full of starch grains. A 

 surface view of the hypodermis, magnified 133 diameters, is shown 

 in figure 4. It presents polygonal outlines, many of wdiich are 

 more or less perfect hexagons. These are the boundaries of the 

 cells of the epidermis. A vertical section (fig. 3) shows at c 

 the same cells in a plane at right angles to the first. They are 

 broad flat plates, very thin walled, several rows deep. They do 

 not contain any protoplasm, which, however, is present in the 

 layers just beneath the bark from d Vo e \x\ fig. 3. The 

 bark upon the parts of plants which are in the air is commonly 

 composed of broad and flat cells devoid of protoplasm, and with 

 their outer walls very much thickened (to prevent evaporation) 

 and (as explained by Bessey in his Botan}-, Advanced Course, pp. 

 93-94) generally only one cell-layer deep. In aquatic plants the 

 thickening of the outer wall is not extensive, if present at all. 

 The bark in the potato is not exposed to the drying influences of 

 the air. being enveloped with moist earth, and hence wx find no 

 thickened cell-walls. Its cells as figured are only the shells of 

 the living units which did their life-work in forming the protect- 

 ive brown and somewhat scaly envelope about the cells within 

 and from which the protoplasm has departed. The brown color 

 of the cell which gives the characteristic color to the potato peel 

 is interesting. In sections the color shows not as any definite 

 grains of pigmentary or coloring matter, but as a diffused yellow- 

 ish tinge seen in all parts of the epidermal cell-wall. 



This color is due to the presence of a very slight amount of the 

 same substance which is present abundantly in the cells which 

 form hard coverings like the hard bark of a tree or the shell of a 

 nut, and it shows that the epidermal cells of the potato, when alive, 

 secreted such matter as the epidermal cells of other plants often 

 secrete upon a much larger scale. I do not think that this secretion 

 can be said to be of any use to the potato. It is not extensive enough 

 to perceptibly thicken the cell-walls, though it may be tougher 

 and a better protection than ordinary cellulose would be. If it is 

 not of any use its presence may be a case of persistence of the 



