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 part 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 want 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’. 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 Zransforming 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. III. 
