396 
Blackman and Welsford. — Studies in 
dark coloration, which is one of the characteristic signs of death in the 
bean leaf, gradually spreads through the leaf. Text-fig. i is a diagram of 
a leaf in which the epidermal cells only are infected, but one hypha 
has grown down and come in contact with two palisade parenchyma 
cells. All the palisade parenchyma cells below the infected epidermal area 
are however discoloured, thus showing that the death of these cells has 
taken place in advance of the fungus. Text-fig. 2 shows a leaf in which 
the fungus has penetrated to the spongy parenchyma cells, but the dis- 
coloration of the tissues has spread through to the lower epidermis. This 
discoloration may not be solely the result of the enzyme diffusing out in 
advance of the invading hyphae, but may be partly due to action of 
substances liberated from the dead or dying cells. 
The action of Botrytis on the epidermal and parenchyma cells respec- 
tively is somewhat different. In the epidermal cells the walls usually swell 
before the protoplast shows much alteration. In the mesophyll, on the con- 
trary, the first morbid change is seen as slight disorganization of the 
protoplast ; the swelling of the wall is not noticeable till a later stage. 
It is of course possible that changes occur first in the cell wall but are 
overlooked owing to its thinness. 
Summary. 
The early stages of infection by Botrytis drier ea of the leaf of the broad 
bean ( Vida Faba ) have been studied. The spores were grown in drops of 
turnip juice on the leaf. 
When the germ tube produced from a spore has reached a length 
of about one spore-diameter it can be shown to possess a mucilaginous 
investment. By means of this sheath it becomes firmly fixed to the 
substratum. 
The germ tube exerts a considerable pressure on the underlying leaf 
tissues, as is shown by the slight depression of the epidermal wall below it. 
Actual penetration of the leaf is usually brought about by the develop- 
ment of a fine peg-like outgrowth from that part of the germ tube which is 
firmly pressed against the leaf surface. 
Penetration can occur without the development of an appressorium. 
Prior to the penetration of the cuticle no softening, nor swelling, nor 
any other change can be observed in the cuticle itself or in the underlying 
layers of the epidermal wall. The piercing of the cuticle is due solely 
to the mechanical pressure exerted by the germ tube as a whole or by the 
special outgrowth from it. Such a method of penetration would clearly be 
impossible in the absence of the mucilage which holds the germ tube firmly 
in position and enables it to exert the appropriate pressure. 
As soon as the germ tube has forced the cuticular barrier enzyme 
action can occur, as is shown by the swelling of the subcuticular layers. 
