G30 



ligations have gone, no fungus exists in the early stage of the disease. 

 We now arrive at the most important part of the plant in an eco- 

 nomical point of view, that modification of the stem in which starch 

 is so freely deposited, and which is thereby rendered fit for the food of 

 man. Before we discuss the nature of the disease in the tuber, it is ne- 

 cessary to pass in review the structure of this part of the plant. The 

 tuber, it is almost unnecessary to say, consists of a simple enlargement 

 of the underground stem, by the enormous development of cellular 

 tissue, containing an immense multitude of starch granules. Hence 

 we may expect to find in it all the ordinary structures of the stem 

 very much separated and as it were disturbed by the excessive deve- 

 lopment of the pith, but differing from the stem in the almost if not 

 total absence of woody fibre. If we make a perpendicular section of 

 the tuber and look at the cut surface, we see two lines diverging 

 from the point of entrance of the stem, and which pass round the 

 potato and meet again at its opposite extremity, where the under- 

 ground stem was again continued of its ordinary size. These lines 

 are indications of the woody tissue of the stem, the large circle they 

 surround being the dilated and enormously developed cellular 

 structure of the pith filled with starch granules. Along the central 

 axis of the potato we observe a dull line, which only differs 

 in its microscopic structure from the remainder of the cellular portion 

 of the tuber, in perhaps containing fewer starch granules. This dull 

 line marks the normal development of the pith in the ordinary stem. 

 The epidermis is separated from the bundles resembling and continuous 

 with the woody tissue, by a considerable interval filled up by similar 

 cellules, filled with starch. The cells are for the most part of an 

 hexagonal or pentagonal outline, and filled, or nearly so, with starch 

 granules, except near the surface, where they are more compressed 

 and contain less starch. The vascular bundles, which in a transverse 

 section of the tuber form a circle, consist of barred or dotted ducts, 

 with a very few spiral vessels, and surrounded by compressed or pris- 

 matic cellular tissue and little or no woody fibre. The walls of the 

 cellular tissue are perfectly transparent, colourless and slightly gra- 

 nular, buds are developed on points of its outer surface ; such is the 

 condition of the tuber in its healthy state. 1 must more especially 

 remark that nearly the whole of the cellules, except those immediately 

 surrounding the vascular bundles, and beneath the epidermis, are ^/fefZ 

 with starch granules, which occupy the whole of the cavity, some few 

 of the cells being less crowded with starch granules. When we examine 



