212 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



the body of its host plant are at least two-fold in nature. First 

 there is the attraction, which directs the attack, and secondly there 

 is the actual method of penetration. It is probable that in all cases 

 the sense of direction is given by some substance that exudes from the 

 host plant, and that this serves as the means of attraction which 

 determines the direction of growth towards it on the part of the 

 parasite. An excellent example of this is presented by germinating 

 pollen grains. The pollen tubes, which convey the fertilizing material 

 to the ovule, are indispensable to the setting of good seed in the 

 majority of the higher plants, nevertheless the pollen tubes them- 

 selves are to be regarded as being parasitic on the tissues which they 

 traverse. They feed at the expense of the tissues through which 

 they grow, just as do the mycelial hyphse of a fungus. They are 

 guided, in their first entrance through the stigma; by the exudation of 

 a sugary substance which is emitted from the superficial cells of that 

 organ. The presence of this sugar determines the direction of growth, 

 as may be easily proved by allowing pollen grains to germinate on the 

 surface of agar-agar. At first the tubes grow out in all directions, 

 but if a piece of ripe stigma is placed on the slide the tubes will soon 

 deviate from their indifferent paths, and will grow towards the source 

 — i.e. the stigma — in which the stuff that excites their irritable move- 

 ment is most abundantly present, and. from which it is diffusing 

 through the agar. The annexed figure (Fig. 46) presents a photo- 

 micrograph of pollen tubes of the common Bluebell [Scilla nutans Sm.) 

 which are thus turning chemiotactically, as it is termed, towards a bit 

 of stigma. The hyphae of fungi are endowed with the same faculty of 

 chemiotaxis, which, indeed, is a very widespread phenomenon among 

 plants, and is to be also discerned in the way in which roots will grow 

 towards and invade richly manured land. Sometimes, however, the 

 parasite itself emits the chemiotactic substance, and then it causes its 

 prey to grow towards itself. In other instances, e.g. the Broomrape, 

 the seeds will not germinate unless they are in close proximity to the 

 roots of a suitable host plant. 



What we call predisposition on the part of a plant to disease is 

 probably largely a matter of inability on the part of the plant exposed 

 to infection to prevent the exudation of chemiotactic substances, 

 which thus guide the hyphae of a germinating fungus spore and cause 

 them to penetrate the epidermis of the plants they infect. 



Immunity from attack may be due to a variety of causes. A 

 thick cuticle, besides preventing the exudation of chemiotactic 

 substances, often prevents the entrance of the invasive hyphae, 

 because they have no means of penetrating it. Entrance is then 

 effected either through natural orifices, the stomata, or through 

 wounds, as is perhaps most often the case with fungi such as those 

 responsible for the Larch disease (Peziza Wilkommii) or the apple and 

 beech canker [Nectria). There is a considerable body of evidence 

 to show that the wounds produced by animal sucking-pests, e.g. 

 the Beech coccus and the American bhght, furnish the ordinary means 



