Parasitism. 



CHAPTER XVI. 

 THE ORIGIN AND SPECIALISATION OF PARASITISM. 



In a group of parasitic fungi like the rusts the question naturally arises, 

 How did this parasitic habit originate? Parasitism in fungi is an adapta- 

 tion whereby the fungus can directly draw its nourishment from the living 

 material, and in order to do this it must have become accustomed to a 

 new mode of life, for it is assumed that originally the fungi obtained the 

 requisite substances for food from dead or decaying organic material. 



The saprophytic mode of life seems to have led up to the parasitic, tor 

 there is every sort of gradation between the two. Some parasites are able 

 to complete their development entirely on artificial nutritive media. Others 

 require to infect the living plant first, and then undergo their final develop- 

 ment on dead tissue, while others begin their life on dead material, as a 

 preparatory stage to passing over to the living substance. 



The origin of parasitism and that of specialisation are so intimately 

 bound up that they may be conveniently considered together, for at the 

 critical moment, when the spore first put forth its germ-tube into the living 

 tissue of a particular host-plant and was able to grow there, then parasit- 

 ism was established, and if the fungus confined itself to that host then 

 specialisation had begun. 



What induced the fungus to enter the living plant by means of its 

 germ-tube, and afterwards confine itself to one or a few closely-allied 

 species is the question to be answered. It does not seem difficult to account 

 for the entrance of the germ-tube into the stoma, for it follows the lines 

 of junction of the cells, and ultimately comes to a stoma, into which it 

 dips just as it would into any other opening. But to be able to penetrate 

 the cells and abstract nourishment from them is the point which requires 

 explanation. 



It is assumed that the saprophytic habit was the normal one among 

 fungi, and that parasitism is an acquired habit. Massee 9 claims to have 

 proved this assumption to be a fact, for he says " A saprophytic fungus 

 can be gradually educated to become an active parasite to a given host- 

 plant, by means of introducing a substance positively chemotactic to the 

 fungus into the tissues of the host. By similar means a parasitic fungus 

 ran be induced to become parasitic on a new host." Parasitism, then, is 

 flue to chemotaxis, which is a form of sensitiveness in the plant whereby it 

 has an affinity for certain substances, and is opposed to others. Thus there 

 are various substances which are capable of attracting or repelling the germ- 

 tubes of fungi, and the name of positive or negative chemotaxis has been 

 given to this property. 



In an extensive series of experiments conducted with both parasitic and 

 saprophytic fungi, Massee 9 has shown that certain substances in the plant 

 are positively chemotactic in their nature, and others negatively so. Thus 

 it was found that sugar is the most general of positive chemotactic sub- 

 stances, although its action on the germ-tubes of obligate parasites is very 

 slight. Experiments showed that " it was not sufficiently powerful in any 

 instance to attract the germ-tubes through perforations in mica or through 

 stomata." 



Specialisation of Parasitism. 



Recent investigations in connexion with heteroecious rust-fungi, or those 

 which change their hosts and produce a different kind of fungus on each 



