298 THE PARASITIC DISEASES OF PLANTS. 
The above list contains only a few of the very large number c 
known fungoid diseases of plants, but will serve the purpose c 
showing their variety. The fungi that produce these diseases ar 
by no means closely related to each other. The higher fungi ar 
divided into many classes and the disease-producing parasites ar 
distributed among them all. For these distinctions, however, th 
student must be referred to books upon botany. 
THE BACTERIAL DISEASES OF PLANTS. 
Only within recent years has it been appreciated that bacteri 
are important agents in producing plant diseases. Even afte 
their agency in causing diseases in animals had been fully recog 
nized it was denied that they could produce troubles of this soi 
in plants. Up to very recent date, it was claimed that it was a: 
impossibility for bacteria to penetrate plant tissue so as to produc 
trouble. Plant cells are provided with hard cell walls of cellulos 
and wood, which protect the living protoplasm within; and, sine 
these cells form the bulk of the plant and are adherent to eac' 
other, it was difficult to see how bacteria could penetrate into th 
plant at all. The mycelium of the higher fungi can do this readil 
since it can thrust itself between the cells, and thus grow easil 
within the solid tissues; but it seemed impossible to believe tha 
bacteria could penetrate the hard tissues. Within recent yean 
however, it has been demonstrated that this is possible and th 
last ten years have disclosed many bacterial diseases of plants 
until to-day we know of more bacterial diseases of plants than o 
animals. 
The Black Rot of Cabbage (Pseudomonas campestris) . A: 
illustration will best show the general course of such a disease and a 
the same time indicate how conclusive is the proof of the agency o 
bacteria. For this purpose will be chosen the black rot of th 
cabbage, cauliflower, turnip, and several other members of the famil 
Cruciferce. The disease appears first, as a rule, upon the edges o 
the leaves, as brown spots, that spread down the leaves following th 
veins to the midrib and petiole and finally into the main stem of th 
