416 H. MARSHALL WAKD. 



hypha is to be seen between the cells of the spongy mesophyll 

 close to the epidermis at this part : it is cut through. The 

 dark grey cell below the smaller hair is filled with fine 

 granular particles of calcium oxalate. Below and to the right 

 of this cell a hypha is making its way in the spaces between 

 the palisade cells to the stoma (in oblique longitudinal sec- 

 tion) on the morphologically upper surface of the leaf. It 

 will be noticed that the cell walls of the epidermis turn brown 

 over the badly infected portions, but, as elsewhere, the dis- 

 colouration and disorganisation do not result immediately on 

 the access of the mycelium ; the process is accelerated or re- 

 tarded by wet or dry weather — possibly apart from the mere 

 quantity of mycelium, though certainly not independent of its 

 presence. 



Fig. 3. — Portion of the tip of a branch of one of the conidio- 

 phores which had grown out from a piece of potato-tuber, 

 lying in a drop of water beneath the microscope. The order 

 of development is from a to e ; the conidium marked "^ is the 

 same throughout. In a the tip of the hypha is commencing 

 to swell up, and soon becomes an ovoid body full of fine- 

 grained protoplasm (6), still continuous with that of the coni- 

 diophore. Soon afterwards the now larger conidium presents 

 a more granular clouded appearance, and its proximal or basal 

 end is cut off from the hypha below by a septum. At its distal 

 end a colourless papilla has appeared, due to the deliquescence 

 and swelling of the cellulose wall at this point. In d this 

 conidium is nearly ripe, and in e it has just fallen away by the 

 rupture of the short, slender pedicle. Meanwhile, as the coni- 

 dium referred to was approaching maturity, the hypha con- 

 tinued to grow by bulging out below the septum in c, and (in 

 this case) to the right. This pushed the point of union between 

 the above conidium and its hypha over to the left, and thus 

 caused the displacement of the conidium in d. In the latter 

 figure the new growing point of the hypha has already swollen 

 up to form a new terminal conidium, which was in its turn 

 displaced (to the left also), as shown in e, by a renewed growth, 

 and so on. In each case a flask-like dilation of the hypha 



