PLANT IN RELATION TO ITS BIOLOGICAL ENVIRONMENT. 213 



for the entrance of the fungal disease. When once inside the host 

 plant, a fungus often attacks special tissues, mainly perhaps because 

 its special ferments enable it to utilize certain constituents of the plant 

 body more easily than others, but partly because it may be restrained 

 from entering the cells of other tissues. Each case has to be studied 

 for itself; and even closely related species, e.g. of Nectria, exhibit 

 considerable differences in the form of attack on the interior tissues 

 of the plants they may have invaded. 



The limitation to special external layers of the cortex which is to 

 be seen in the case of many mycorrhizal fungi probably indicates some 

 prophylactic power resident in the cells of more internal layers. That 

 plants are able to restrain the growth of fungi is shown by a study of 

 nectaries, both floral and extra-floral. I have been unable to find 

 that any special investigations have been made by others as to why 

 nectaries, with their exposed sugary surfaces, should be practically 

 imnmne from the attack of moulds and yeasts. But the results of 

 experiments which I am now carrying on seem to point to the presence 

 of some inhibiting substance which prevents the spores from even 

 germinating on a medium which might have been expected to be very 

 suitable for such a process. 



The presence of cuticle undoubtedly prevents the entrance of some 

 pests. Thus the germ tubes of Botrytis, according to experiments now 

 in progress at the Imperial College, are unable to attack it ; that is 

 to say, they secrete no ferment which is competent to dissolve it. 

 But if they once obtain an entrance, through a wound or in other ways, 

 they rapidly act on the middle lamella, and bring about a speedy 

 disorganization of the tissues. It has been found possible to extract 

 this active substance and examine its disintegrating effects on a variety 

 of plant tissues, and thus to elucidate the way in which this pest 

 brings about the death of its host plant. 



Many parasites, especially those of a more advanced type, in- 

 fluence their hosts in another way. They do not kill, at least not 

 at once, but they interact with their host in various remarkable 

 ways, producing definite kinds of growths or galls. Amongst the 

 flowering plants, the tropical parasites belonging to the order Balano- 

 phoraceae attack the roots of other Angiosperms. The parasite itself is 

 greatly reduced ; it would scarcely be recognized as a flowering plant 

 at all, for it consists at first merely of an undifferentiated mass of 

 cellular tissue within the body of its host. It causes copious branching 

 of the root of its victim at the seat of infection, the whole forming a 

 tuber-like body, and consisting of a crowd of rudimentary roots all 

 interpenetrated by the cells of the Balanophora. Only later is a 

 flowering stem formed within this mass of cellular tissue, from which 

 the parasite finaUy bursts forth as a many-flowered inflorescence of 

 relatively large size and remarkable form. Many of the fungi belong- 

 ing especially to the rusts and smuts produce exuberant growth on the 

 part of their hosts, and I will instance especiaUy the species known as 

 Ustilago Treubii, which infects Polygonum chinense in Java. It causes 



VOL. XL. Q 



