432 



BOTANY 



PART II 



A 



haustoria leads to the discoloration and death of the foliage and tubers. Sexual 

 reproductive organs have not as yet been observed in this species on the Potato 

 plant but have been produced when the fungus is cultivated on certain media. 

 Asexual, oval sporangia are formed on long branching sporangiophores which grow 

 out of the stomata, particularly from 

 those on the under side of the leaves 

 (Fig. 374), and appear to the naked 

 eye as a white mould. The sporangia, 

 at first terminal, are cut off by trans- 

 verse walls from the ends of the branches 

 of the sporangiophore, by the subse- 

 quent growth of which they become 

 pushed to one side, and so appear to 

 be inserted laterally. Before any 

 division of their contents has taken 

 place, the sporangia (B) fall off and 

 are disseminated by the wind ; in 

 this way the epidemic becomes wide- 

 spread. The development of swarm- 

 spores in sporangia is effected only in 



water, and is consequently possible 

 only in wet weather. In this process 

 the contents of the sporangium divide 

 into several biciliate swarm -spores 



FIG. 373. Achlya polyandra. The fertilisation 

 of two egg-cells, o, of an oogonium by two 

 tubes from the antheridium, a ; ek, nucleus 

 of the egg-cell ; sk, sperm-nucleus ; in o 2 

 the section has not passed through the egg- 

 nucleus. (After TROW.) 



FIG. 374. A, Surface view of the epidermis of a 

 potato leaf, with sporangiophores of Phyto- 

 phthora infestans projecting from the stomata 

 ( x 90) ; B, a ripe sporangium ; C, another 

 in process of division ; -D, a swarm - spore. 

 (B-D x 540. After STRASBURGER.) 



(C, D}. Each of these spores, after escaping from the sporangium, gives rise to a 

 mycelium, which penetrates the tissues of a leaf. The sporangium may also ger- 

 minate directly without undergoing division and forming swarm-spores. A similar 

 transformation of sporangia into conidia is also found in other Peronosporeae as a 

 result of their transition from an aquatic to a terrestrial mode of life. 



Plasmopara viticola, an extremely destructive parasite, also produces copiously- 

 branched sporangiophores and occasions the "False Mildew " of the leaves and fruit 

 of the Grape-vine. Albugo Candida (=Cystopus candidus], another very common 

 species, occurs on Cruciferae, in particular on Capsella bursa pastoris, causing 

 white swellings on the stems. In this species the sporangia are formed in long 



