: oe 
9 
In these spaces between the loosely arranged cells the air ¢irculates 
freely. No spaces occur between the cells formin g the opami but 
special mouths or stomata exist, through which vaa air can enter, and 
‘hee, gireolate through the tissues of the leaf, sro., Fig. 
of the utmost importance for the health of the plant that these 
SARRO zep be kept open, so that the air can have ready access 
to the cells of th . 
e number of stomata through which the air p is very great, 
as — as 70,000 exist upon one square inch of onion 
Having now considered the structure of the leaf of ee onion, we can Structure and 
return to the fungus which lives upon it, and then the importance of Perondi Mi of 
this preliminary description will become evident iis Sehid- 
the surface of a small piece of the leaf of a diseased | nian 
onion plant examined under a microscope. ‘The outlines of the cells 
forming the epidermis are shown at EP,; sTO. represents one of the 
l At 
will be seen that the opening of the stoma is i heska by a i Tike 
i e 
t its outer end this tube splits into many branches, and at the end 
of some of the branches is a little oval body, which hangs from it like 
a drop of water. These little bodies are the Spores, and they sa as 
seeds. It is by their means that the fungus is able to reproduce itself. 
Some of the spores have fallen off from the ends of the branches, and 
are lying on the leaf (sp.). 
This ching ih with its spores is the mature fungus Peronospora 
Schleideniana, and is is the collec tion of these which causes the powdery 
appearance on the 
ere is a slight but distinct violet colour seen both in the fungus and 
its spores. 
If a thin slice or section of the diseased onion leaf, which passes 
directly through one of the stomata, be examined, it will be possible to 
follow the stem of the fungus through the patna into the tissue of 
the leaf and see what comes of it. 
uch a section is represen to’ at Fig. 3; sto. is the stoma, through 
which emerge in this case two stems. ese branch at their ou uter 
ends, and some of ‘ie branches still bear spores ; thers, however, have 
lost them. The lower ee each stem can be traced through the 
ro ot-like proc cesses (TP 7 which in their turn branch again, and thus, 
by continually dividing, a meshwork of tubular roots is formed which 
passes in every direction through the spaces bereen ls. 
The injury which the fungus does to the onion plant is chiefly due to 
this network of roots, and it is of two ind, irstly, by blocking up 
sure Secondly, za kagi ka upon the substance of the plant, 
absorbing, by means of its meshwork of bala? roots, the nutritive 
e very intimate relation which exists between the fungus and the 
The 
tissue of the leaf is most noteworthy. The tubular roots pass every- 
close connexion will be more evident later When remedial measures are 
ussed, 
dise 

