rx.] MILDEW OF ONIONS. 47 



the evaporation of water in the form of vapour from the 

 plant attacked. So potent are the different members of 

 the genus for evil that their spawn threads are capable of 

 pushing aside the plant-cells of the plants attacked either 

 from without .or within. The mycelium of some species 

 has also the power of piercing through the cell-walls and 

 traversing the interior of a plant, not by creeping be- 

 tween the cells, but by breaking down the cell-walls in 

 its progress. Putrefaction attends the whole progress of 

 growth of the invading Peronospora. The complete tree- 

 like habit of the fungus of onion mildew is shown in 

 Fig. 15. It will be noticed that the fungus repeatedly 

 branches and rebranches, and at the ends of all the 

 minor branchlets the ovate spores or acrospores termed 

 conidia are borne. These are shown at B, and a single 

 conidium is farther enlarged to 1000 diameters at C. 

 The conidia are pale gray or pale lavender in colour, and 

 are very large in comparison with the conidia belonging 

 to other species of the genus Peronospora. At the time 

 of germination the spores usually burst at the side. At 

 Fig. 16 part of the Peronospora is enlarged to 400 

 diameters, so that the different illustrations of the genus 

 in this work may be presented uniform in size. The 

 difference in size between Peronospora Schleideniana, Ung., 

 Fig. 16, and P. exigua, W.Sm., Fig. 2, is very great. 

 From the irregular mass of protoplasm exuded at the time 

 of germination mycelial threads and fruiting branches 

 quickly arise. The fungi of clover mildew are trans- 

 parent and almost colourless in all their parts, whereas 

 the onion fungus is more or less tinted with a pale 

 reddish-gray, a brownish, or a dull violet hue throughout. 

 This is especially noticeable in the comparatively large 

 conidia. The Rev. J. E. Vize, of Forden, Welshpool, 

 has found the resting-spores of this species in decaying 

 patches upon onions, where the Peronospora in its conidium- 

 bearing state previously existed. The oogonium, or unim- 

 pregnated oospore, or resting-spore, is shown at D (Fig. 1 5), 



