xxxv.] POTATO DISEASE, I. ITS ACTIVE STATE. 283 



must at once place it under the higher powers of the micro- 

 scope. If the slicing through a disease spot is successful, we 

 shall probably see the atom when magnified 100 diameters, 

 as at Fig. 127. The thickness of the lamina of the leaf 

 is shown at A, B ; the under side of the leaf is represented 

 at A, from which surface the fungus almost invariably 

 springs. The fungus, therefore, really grows downwards. 

 The true upper surface is shown at B. This reversal 

 of the leaf in the illustration is merely, as in other in- 

 stances in this book, to show more clearly the treelike 

 branching growth of the fungus. If we confine our 

 attention for the present to the section of the leaf, we 

 shall note that it is made up of minute bladder-like cells, 

 loosely packed together ; and that the cells at top and 

 bottom, representing the lower and upper cuticle of the 

 leaf, are devoid of the shading, which is meant to indicate 

 the green colouring matter or chlorophyll within. An 

 opening into the interior of the leaf will be seen at C ; 

 this is one of the stomata or organs of transpiration, 

 sometimes referred to as " breathing pores." The stomata 

 are like the gates to a camp or to an entrenched position ; 

 they are the weak points through which an enemy may 

 enter, and when once these gates are passed, the whole 

 interior of the plant is at the mercy of the invader. At 

 D may be seen a hair built up of four transparent cells, 

 the two lower being traversed by a mycelial thread of the 

 potato fungus. On the upper part of this hair, attached 

 to the outside at E, may be seen one of the small branches 

 of the fungus ; this branch has burst and thrown out a 

 mycelial thread from its side. Every fragment of the 

 potato fungus is capable of growth, and of ultimately 

 reproducing the parent fungus. The cells immediately 

 under the true upper cuticle of the leaf at F are termed 

 pallisade cells ; and their disposition in the manner 

 illustrated serves to give the necessary firmness to the 

 exposed upper surface of the leaf. 



If we now look within the fragment of the leaf we 



