178 PLANT LIFE 
the “constitution” of a plant, it is also 
affected by the influence of surrounding 
conditions of life. 
We know very little, as yet, about the 
nature of “constitutional” resistance. In 
some cases it depends on a well-developed 
epidermis, and on the absence of attractive 
substances, such as sugar, from parts of the 
plant readily accessible to the fungus. A 
curious example of immunity against rust 
fungi is furnished by some of the cereals 
recently raised at Cambridge. The fungus 
normally gains access to the interior of the 
leaf by the germ tube or hypha growing in 
by way of a stoma, and then attacking the 
living cells. But it is possible to find a 
wheat plant so sensitive to the influence of 
the fungus that the cells die immediately the 
hypha approaches them. The fungus is thus 
effectively starved, and is unable further to 
infect the plant. It may also happen that 
when a fungal hypha has entered a plant, the 
attacked and injured tissues are cut off from 
communication from the healthy ones by 
a layer of impervious cork, and in this way 
the further spread of the disease within the 
body of the plant is prevented. 
The part played by the environment in 
increasing liability to infection depends on a 
number of possible factors, all of general 
biological interest. A close damp atmosphere 
is not only favourable to the germination of 
the fungus spore, but it may at the same time 
