CHAPTER VII. — PHENOMENA OF VEGETATION. — PARASITES. 367 



described in the following paragraphs and will be illustrated by examples. Some of 

 them have been already noticed in passing in the sections of Chapter V, where they 

 will be readily found with the help of the index. Further details must be sought in 

 the different monographs and in pathological treatises. 



Among the phenomena which are of quite general occurrence it may be 

 mentioned in connection with the growth of the parasite, that in extreme cases it 

 either continues to be confined to the spot where it first attacked its host and to its 

 immediate neighbourhood, or spreads far beyond that spot ; in the latter case 

 it may grow through or over the existing parts of the host for considerable dis- 

 tances, or pari passu with the growth of the host, as is specially seen in many 

 Lichen-fungi. In smaller hosts consisting of one or few cells, with the exception of 

 the Lichen-fungi which will be described at length in the sequel, the difference 

 between these cases is of course small; in larger plants on the contrary it is very 

 striking. The Laboulbenieae, for instance, which are parasites on insects are narrowly 

 confined to the part which they first attack ; the species of Cordyceps which belong to 

 the Entomophthoreae grow through the entire body of the insect. Many corresponding 

 examples might be mentioned from parasites on plants, and it need scarcely be added 

 that there is no w r ant of intermediate forms between the two extremes. 



Parasites which spread through the whole of the host, or over large portions of it, 

 may either show the same behaviour and the same development on every or almost 

 every part of the body, or they may have certain phases of their development confined 

 to certain parts, and this latter rule may be invariable or be very generally observed. 

 Parasites on insects, species of Cordyceps for example, spread almost through the 

 entire body of the creature ; C. militaris puts forth its stromata at any part 

 without distinction of the surface of the caterpillar which it attacks, often at many 

 places at the same time ; C. sphecocephala only on the under surface of the thorax 

 between the first or between the two first pairs of feet of the West Indian wasp 

 (Polistes Americanus) which is its host 1 . The same rule will be exemplified below 

 in the case of very many parasites on plants, and has been already noticed to some 

 extent in Chapter V. 



According to their effect on the host and the reactions of the host on this 

 effect, two chief classes of parasitic Fungi may be distinguished, namely, a destructive 

 and a transforming or deforming class ; the two extremes are united by a large 

 number of intermediate forms. 



When a parasite of the destructive class attacks and occupies its host the parts 

 attacked by it become sickly, die, and are decomposed in a longer or shorter time 

 without previously showing any signs of abnormal growth. It depends on the 

 particular species whether these phenomena in large plants are confined to the parts 

 directly attacked by the parasite or whether the whole body of the host becomes sickly 

 and dies. All facultative parasites may be placed in this class, as will be shown in detail 

 below; of obligate parasites on plant-forms the species of Phytophthora almost 

 without exception, many Uredineae, such for example as the species of Puccinia which 

 live on the Gramineae, or at least those portions of their life-cycle which inhabit 

 the grass, and with some exceptions the Ustilagineae (species of Tilletia, Ustilago 



1 See Tulasne, Carpol. Til. 



