488 H. MARSHALL WARD. 



Still only partially injured, the hyphse are seen to bore through 

 the cell-walls, cross the cell-cavities, and send out secondary 

 branches in all directions, which repeat the same processes in 

 their turn. 



If such a piece of " infected " tissue be placed in water in 

 contact with a healthy seedling of cress for 12 — 24 hours, the 

 latter will be found firmly attached to the former by means of 

 hyphee which have grown across the interval and commenced 

 to bore their way into the healthy tissues. These penetrating 

 hyphae not only enter any stomata in the epidermis of the 

 attacked plant, but make perforations through the cell-walls 

 as before (fig. 2). Similar events follow if a portion of the 

 infected tissue be placed on the clean, cut surface of a potato 

 (fig. 3), or on portions of many other plants. These facts will 

 be referred to later, when we are concerned with the mode of 

 action of the hyphse. Once inside a suitable tissue, the myce- 

 lium makes it way through all the thin walls as already 

 described. After some hours the tissues thus attacked become 

 reduced to a mere mass of pulp, and the well-nourished myce- 

 lium begins to form its reproductive organs, at the same 

 time developing new ramifications in the surrounding water. 

 The rapidity of these processes, and the extent to which they 

 go on, are dependent on a number of conditions, apart from 

 the nature of the host plant. Amongst these, temperature and 

 the abundance of oxygen are the chief. All being favorable, 

 certain branches of the mycelium become swollen at the apex 

 into pear-shaped bodies, large quantities of protoplasm passing 

 into them. Each of these, having attained a maximum size 

 and become nearly globular, becomes cut off by a septum from 

 the rest of the branch, and persists as a nearly spherical thin- 

 walled cell (fig. 5). 



This is a**conidium" — a simple dilation of the hypha, 

 rich in protoplasm, and capable of germinating at once (in 

 fresh water) after separation from the parent branch. These 

 conidia are formed in immense quantities at the ends of the 

 numerous branches of the mycelium ; not only free in the 

 surrounding water, but also in the destroyed tissues (fig. 2). 



