FUNGOID PESTS OF CULTIVATED PLANTS. 8 



cell is ruptured, and the three or four smaller bodies, which have been 

 differentiated in its interior, escape, each one furnished at one extremity 

 with a pair of delicate movable hairs, by means of which these little 

 bodies, now termed zoospores, can swim actively in any thin film of mois- 

 ture upon which they may fall. Possibly this film may be upon the leaf 

 of a foster plant. In a short time all motion ceases and the zoospores 

 come to rest, the pair of delicate cilia are absorbed, and a germinating 

 thread is produced, the point of which seeks out and enters at one of the 

 stomata of the sustaining plant. Having once obtained an entrance, the 

 thread grows vigorously, and a little mass of threads, called a mycelium, 

 is soon developed within the tissues, capable of spreading itself through 

 the plant which it has infected. In the next stage we discover that this 

 mycelium has developed erect branched threads, which pass out through 

 the stomata again into the external air, sometimes singly, sometimes in 

 tufts. These are the fertile threads of the mould, which soon produce a 

 single conidium at the tip of each of the branchlets, just like the original 

 conidium whence the zoospores were developed (PI. VI. figs. 30, 78). 

 When fully matured each fertile thread produces a score or more of these 

 conidia, which fall away when ripe, and then undergo transformation 

 into zoospores, ready and active, prepared to pass through the same stages 

 again, and indefinitely multiply the pest. This history represents the 

 ordinary conidial fructification of the mould, by means of which it is 

 passed from leaf to leaf, and from plant to plant, until the whole area is 

 affected. How many of the minute conidia may be transported to a con- 

 siderable distance by a breath of wind it is impossible to say, but it is 

 known that they may be carried to any spot where there is sufficient 

 moisture for the conidia to be differentiated into zoospores, and afterwards 

 come to rest and germinate. This process takes place in summer and 

 autumn, but there is yet another means by which the pest is disseminated 

 in spring. 



The mycelium, which flourishes within the substance of the plant 

 infested, is capable of producing larger globose bodies, chiefly within the 

 stems, concealed from external view. These globose bodies secrete a 

 thick envelope, mostly of a brownish colour, and after development they 

 remain in a state of rest within the stems during the winter (PI. IV. 

 fig. 70*). So that old stems of plants, which are infested with the mould 

 during the autumn, conceal within themselves during the winter a large 

 number of these "resting spores." As the old stems rot and decay, the 

 resting spores are set free in the spring, and then a period of activity 

 commences. The contents of these globose bodies become differentiated 

 into a large number of zoospores, which ultimately escape by a rupture 

 of the thick envelope, armed with vibratile cilia, and in all respects like 

 the zoospores which are developed from the conidia. These active zoo- 

 spores swarm over the damp soil, and are carried by the spring rains into 

 proximity with the young seedling leaves of the new crop of host-plants ; 

 then the cilia are absorbed, germination commences, the delicate threads 

 of mycelium enter the nearest stomata, and infection results. In this 

 way, in addition to the spread of the infection from conidia in summer 

 and autumn, provision is made for an attack upon seedlings in the spring. 

 It will be inferred that, in order to check the spread of these diseases, the 



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