SPHAEKOTHECA. 



of gonidia behaving exactly like a single sterigma of a 

 Eurotium gonidiophore on a large scale. The mealy white 

 appearance to which the Mildews owe their name is due 

 chiefly to these gonidiophores, which produce countless 

 gonidia and cause the parasite to spread rapidly from leaf 

 to leaf, and from plant to plant, until very often the health 

 and even the life of the " host " plant are endangered 

 the Hop Mildew (Sphaerotheca Castagnei) sometimes 

 causes great loss to the hop-growers. 



In autumn, or in late summer, should a drought follow 

 a spell of wet weather, the fungus produces small asco- 

 carps, developed in practically the same way as those of 

 Eurotium, but having a much simpler structure. The 

 antheridinm and archicarp (oogonium) arise on separate 

 branches, where these happen to cross each other, and the 

 whole process has been fully worked out. Both these 

 organs grow out from the parent hypha as a short branch, 

 and are cut off by a wall. The oogonial branch enlarges, 

 without division of its single nucleus, but the male branch 

 divides into two superposed uninucleate cells, the lower 

 and longer one being merely a stalk-cell and the upper 

 shorter one the actual antheridium. The two organs fuse 

 at their tips, the male nucleus fuses with the oogonium 

 nucleus, to form a zygote nucleus ; meanwhile from the 

 cell below the oogonium there grow out numerous hyphae 

 which form a sheath, as in Eurotium. 



The zygote (fertilised oogonium) now divides into a 

 lower (stalk-) cell and an upper cell, each with one 

 nucleus ; the stalk-cell develops no further, but the upper 

 cell divides into a row of cells, all except the penultimate 

 one (second from the top) having one nucleus. The pen- 

 ultimate cell has two nuclei, which now fuse, and this cell 

 simply enlarges and becomes the solitary ascus of the 

 ascocarp, the fusion nucleus dividing into eight nuclei, 

 around which the protoplasm collects to form the eight 

 ascospores. From the tissue of the sheath or envelope 

 there arise (1) internal cells forming a nutritive tissue as 

 in Eurotium, (2) external septate hyphae or appendages 

 in some Mildews allied to Sphaerotheca, these hair-like 

 outgrowths or appendages of the ascocarp are hooked at 



