CRYPTOGAMS 



345 



causes the Potato disease, lives in the intercellular spaces of the leaves and tubers 

 of the Potato plant, and by penetrating the cells with its short haustoria it leads to 

 the discoloration and death of the foliage and tubers. Sexual reproductive organs 

 have not as yet been observed in this species. Asexual, oval sporangia are formed 

 on long branching sporangiophores 

 which grow out of the stomata, par- 

 ticularly from those on the under side 

 of the leaves (Fig. 266), and appear 

 to the naked eye as a white mould. 

 The sporangia, at first terminal, are 

 cut off by transverse walls from the 

 ends of the branches of the sporangio- 

 phore, by the subsequent 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 {£) fall off and are dissemi- 

 nated by the wind ; in this way the 

 epidemic becomes widespread. The 

 development of swarm-spores in spor- 

 angia is effected only in water, and 

 is consequently possible only in wet 

 weather. In this process the con- 

 tents of the sporangium divide into 

 several biciliate swarm-spores (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 germinate directly without 

 undergoing division and forming 

 swarm-spores : it then has the value 

 of a single spore cut off from a sporo- 

 phore, and in that case may be re- 

 garded as a conidium. A similar 

 transformation of sporangia into 

 conidia is found in other of the 

 Peronosporeae as a result of their 

 transition from an aquatic to a ter- 

 restrial mode of life. 



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

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

 and fruit of the Grape-vine. Oystopvs Candidas, another very common species, 

 occurs on Cruciferae, in particular on Oapsella bursa pastoris, causing white 

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

 the branches of the mycelium under the epidermis of the host-plant, and produce 

 numerous swarm-spores. 



The sexual organs of the Peronosporeae show, in the manner of their formation, 

 a close resemblance to those of the genus Vaucheria (p. 327'). They arise within the 

 host-plant — the oogonia are either cut off by transverse walls as spherical swellings 



Fia. 266. — A, Surface view of the epidermis of a 

 potato leaf, with sporangiophores of Pkytophthora 

 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.) 



