114 The Besting Sjpores of the Potato Fungus. By W. G. Smith. 
finding them amongst leaves which had been macerated for a long 
period in water. There is, however, nothing iinreasona,hle in fruit 
being perfected in water or very damp places, as it is common in 
the Saprolegniege and amongst Algae in general. To sum up, there 
are four reasons why the bodies here described belong to the old 
Potato disease : 
1. Because they are found associated with the Peronospora and 
upon the Potato plant itself. 
2. Because they agree in size and character with the known 
resting spores of other species of Peronospora. 
8. Because some other moulds are aquatic in one stage of their 
existence. 
4. Because they agree in size with Artotrogus. 
Now that these drawings illustrative of the fungus which causes 
the Potato murrain are reproduced in the following Plates,* it may 
be as well to explain at once some of the terms used and the nature 
and habit of the bodies hereafter referred to, for such readers as 
may not be thoroughly acquainted with the life history of the de- 
structive parasitic moulds to which the Potato fungus belongs. 
For that purpose reference must be made to Fig. 1, which shows 
(greatly enlarged) a transverse section through the leaf of a Potato 
plant ; the two great bodies at A A represent two minute hairs on 
the leaf, and at B B are seen the individual cells of which the leaf 
is constructed. When these hairs and cells are compared with the 
fine thread at C, which represents a branch of the Potato fungus 
coming out of a breathing pore of the leaf, it will be seen how very 
minute the fungus is in comparison with the dimensions of the leaf. 
This fine thread is no other than a continuation of a thread of spawn 
or mycelium which lives inside and at the expense of the assimilated 
material of the leaf. When this thread emerges into the air, as 
here shown, it speedily ramifies in different directions, and bears 
fruit at the tips of the branches, as at D D ; these fruits are termed 
simple-spores, or conidia, because from their smallness they are dust- 
like. It is quite possible they may be an early state of the vesicles 
which contain the zoospores as seen at E, F. However this may 
be, they are commonly arrested in growth when still small, and 
they germinate in an exactly similar manner with the zoospores 
themselves, and may be considered somewhat analogous with seeds. 
The Potato fungus has another method of reproducing itself in the 
" swarm-spores" shown at E, F. These are so called because, on the 
application of moisture (as supplied by dew or rain, or when applied 
artificially), the vesicles set free a swarm of from six to fifteen or 
sixteen other bodies known as " zoospores," so named because they 
are furnished with two lash-like tails, and are capable of moving 
* Plates CXI v., CXV., and OX VI. 
