204 Comparative Physiology. 



leaves, and tlie plants start into a new growth. Sulphur 

 sprinkled on the raildew also banishes it, and is thought to 

 kill the fungus. It appears more in the light of an exudation 

 through the cuticle attracting and feeding the fungi, than an 

 eruptive fever of a substance resembling fungi through the 

 stomata. A cold drought appears to have more effect on most 

 plants subject to mildew on the leaves, as peaches, peas, &c., than 

 a warm drought ; but drought appears the most active agent with 

 most plants in producing this disease. Any state of the 

 weather however, or soil, that will arrest vigorous growth sud- 

 denly, appears to cause it. That it is a real plant and not morbid 

 matter, I think may be proved from the fact that, where peach 

 trees on walls have been much infested with it, it has been 

 found in the soil of the border around the roots ; and the disease 

 could not be banished, in some instances, till the soil was totally 

 removed and the plants washed all over. That fungi will pro- 

 pagate in this way, I have had ample proof in the willows in 

 our nursery, which were attacked by an orange yellow fungus, 

 which in the first year appeared to have been wholly external 

 in its ravages ; but in subsequent years appeared to have been 

 partly, and latterly almost wholly, taken up by the roots and 

 conveyed to the leaves ; as the eruptions occurred principally on 

 the veins and midribs of the leaves, and the cuticle was evidently 

 thrown up outwards and not pushed inwards, as in the first 

 season. The mildew in frames is caused by damp, not drought, 

 and is removed by exposure to dry air. It takes place on the 

 surface, and appears a putrefaction of some substances on the 

 surface of the soil, which communicates to the neck of the plant 

 and destroys it, and does not appear a diseased state of the plant 

 itself. Moisture in excess should be more apt to kill by canker, 

 ulcer, or gum, than by mildew : some gardeners have thought 

 wetness a cause of mildew on peas ; Mr. Knight, however, always 

 considered it to be caused by drought, and I should think it 

 most likely. He calculated 250 millions of seeds from a 

 single mushroom in ninety-six hours. Mildew cannot be ge- 

 nerated by the plant itself, he says : the cause, he thought, 

 was the want of moisture and food to the roots, causing 

 stagnation. If wetness caused stagnation of growth it might 

 end in mildew, but I should think it not so likely. He no- 

 tices the discovery of fungi in yeast, but does not say whether 

 he thinks them a cause or consequence. Liebig ascribes fer- 

 mentation to the presence of nitrogen, and says, it will not 

 commence or go on without nitrogen ; but neither would fungi 

 grow without it ; and, notwithstanding many eminent men as- 

 cribe fermentation to fungi, it appears still doubtful. 



The affinities of the principal divisions of the Vegetable King- 

 dom, he says, may be generally expressed in the following 

 manner : — 



