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 



