10 



The Use of a Simple Microscope. 



the entire bulk of tlie tuber, as may be seen from the following 

 sketch, while the diagram by the side of it shows the condition 



Fig. 1. — Healthy Potato, showing starch 

 in situ. 



Fig. 2.— Diseased Potato, showing absence 

 of starch. 



of a diseased tuber in which the starch is wholly wanting, indi- 

 cating the absence of the power of forming starch as one of the 



features of the disease. An examina- 

 tion of the tuber at many different 

 stages during the growth of the plant 

 may lead to a better knowledge of the 

 means of promoting healthy growth. 



The annexed figure shows a portion 

 of diseased potato with the mycelium of 

 a fungus permeating it in all directions. 

 Starch in wheat differs from that of 

 the potato in outward appearance, and 

 is similarly liable to variation in quan- 



Fig. 3.— Diseased Potato, with fungns. tlty and quality. 



The following diagram (from Que- 

 kett's ' Histology ') shows the relative sizes of different kinds of 

 starch, and also a cell of rhubarb filled with starch-grains in situ. 



Wheat-starch is peculiar in having a number of large, and 

 almost round cells interspersed with a great number of smaller 

 ones of very different appearance, and some few intermediate in 

 size. The large ones are flattened, but the smaller ones are 

 somewhat globose or elongated. 



The granules of oat-starch are compressed like the larger 

 granules of wheat-starch, but have a slight tendency to being 

 angular, although they have no external markings. 



The starches of the dicotyledonous plants — such as beans, 

 peas, vetches, &c. — have a peculiar character quite different from 

 the preceding forms. Those of beans are large, Avith an indented 

 outline, as if about to divide into three or more globular 

 granules. 



