1899J MICROSCOPICAL JOURNAL. 147 



readily falls off, and in a drop of water the contents be- 

 come divided into about eight zoospores, which escape 

 and swim freely by means of their cilia. Soon they 

 come to rest, and if in a suitable nidus, such as the leaf 

 of the plant they prey on, germinate and insert a hypha- 

 tube through a stoma ; this ramifies through the tissue 

 and very soon reaches maturity. Thus in moist weather 

 the plant is reproduced with great rapidity. The two 

 best-known examples of this group are the potato disease, 

 Phytophthora infestans, and the white rust of cabbages 

 and other crucifers, Cystopus candidus, but other species 

 are commonly found on lettuces, onions, various umbelli- 

 fer«3, clover, etc. In many instances a resting-spore is 

 formed in autumn in the decaying leaves and stems. This 

 developes a hard, sometimes rugose, coat, and in this 

 stage remains over the winter, and as the weather becomes 

 warmer, germinates and restarts the cycle. Resting- 

 spores are found in many species of Peronospora, but so 

 far are unknown in the potato blight. 



The "damping off" of seedlings so well known to gar- 

 deners is due to various species of Pythium, which be- 

 longs to a neighbouring group. If some cress seed be 

 sowed very thickly in a pot, some of the young plants 

 will soon be found to bend over and often break off. If 

 one of these weaklings be placed in a drop of water, a 

 crop of Pythium will soon be developed. 



Somewhat allied to these are the curious Saprolegnia?, 

 which are usually found on animals, insects, etc., in water, 

 one species causing the well-known "salmon disease." 

 In these we have two forms of fructification, zoosporan- 

 gia, which give rise to swarm-spores, and oosporangia, 

 containing oospores, which are fertilized by spermato 1 - 

 zoids produced in antheridia, which are branchlets special- 

 ised for the purpose. 



In the black and white moulds, found on all kinds of 

 decaying substances, we have a very great variety of 



