114 VEGETABLE PAEASITES. 



which spread themselves on the surface of certain organs have 

 time to abstract them for their own use. This is the common 

 requirement of all parasites, and, just as those which live on 

 other plants are chiefly met with on the epidermis (of the leaves 

 or bark), which is remarkable for its slow and weak assimilation, 

 so we find those living on animals preferring those parts which are 

 slowest in changing their elements as scales, shields, wing- 

 covers, shells of muscles, epithelium, epidermis, &c. ; or they occur 

 in tedious diseases which are followed by weakness, or a retarded 

 and slower reproduction when the molecules of the tissue appear 

 to have been retained too long. On animal mucous membranes 

 such formations which undergo a slow metamorphosis are pro- 

 duced upon the epithelium in spurious membranes, or by a 

 diseased, acrid mucus. Similar changes are observed when a 

 retardation in the change of elements takes place, in consequence 

 of the wearing out of the spinal marrow in the Batrachia, when 

 Saprolegnia ferox begins to spread most luxuriantly. Deposits 

 of food which stick to the teeth, or, in certain insects, to the 

 folds of the mucous membrane of the peritoneum, and which 

 undergo in these places a retarded metamorphosis, produce simi- 

 lar effects. Here we have the analogue of the great theory of 

 " manuring" without which agriculture could not exist, nor could 

 vegetable parasites thrive. This constitutes, also, the principal 

 difference between the nourishment of animal and vegetable para- 

 sites. The former live upon the fresh juices and supplies of their 

 host, which they first decompose or assimilate ; the latter live on. 

 and take up their food from substances already in a state of 

 decomposition. When the spores of the vegetable parasites be- 

 come once fixed, they take their food either externally from the 

 medium which surrounds them which rarely happens with the 

 vegetable parasites found on man ; or their presence causes the 

 soil (i. e. } the tissues) to be saturated with a peculiar fluid which 

 changes in the air or not, and which may lead even to suppura- 

 tion. All this greatly favours the rapid growth of Fungi. We 

 have a good illustration in muscardine, where the animal itself 

 (the silk-worm) shows the first signs of disease, and as soon as 

 the fungus known under the name of Botrytis has fixed itself, the 

 circumstances favorable to its development are increased by its 

 very presence. Artificial vaccination of vegetable parasites is the 

 more successful the more diseased the animals are which are 

 employed (see the experiments of Hilling and Hannover on 



