M'Nab — On Apospory in the Thallophyta. 467 



moniliform series, a condition which, however, is also met with in 

 the allied Monohlepharis polymorpha. 



We may therefore regard the so-called conidia or spores in the 

 Peronosporese as detachable sporangia, dissociation of the sporan- 

 gium and sporophore being of importance in the scattering of the 

 ciliated zoospores which takes place under favourable conditions. 

 Many modifications occur in the formation of the zoospores, as 

 fully described by Yan Tieghem {Traite de Botanique, p. 1022), 

 and while in many instances zoospores are normally produced, 

 under other conditions they are always wanting : while in a third 

 series zoospores may or may not be produced. When the spores 

 are not produced, and direct development occurs, we have un- 

 doubted examples of apospory. This is best shown in CystopUs 

 portulacce, and in Phytophthora infestans (Yan Tieghem, loc. cit.). 

 In Cystopus piortulaccB the first and oldest sporangium of the 

 moniliform series behaves differently from the others of the same 

 series. The lower ones all develope as sporangia, and liberate 

 zoospores; but the upper one does not liberate its contents, and 

 sending out hyphae, developes directly into a new thallus. This 

 is an example of apospory. Phytophthora infestans exhibits 

 apospory when the sporangia are grown in moist air, as hyphae 

 are directly produced ; but zoospores are developed in the usual 

 way when the same sporangia are placed in water. Lastly, in a 

 number of species of Peronospora a single spore may be formed, 

 and escape from the sporangium, after which it germinates in the 

 usual way ; or else the sporangium sends out hyphae, and the 

 spore is suppressed. This is seen in Peronospora gangliiformis 

 P. parasitica and others : all of which are to be regarded as truly 

 aposporous (see Yan Tieghem, Traite de Botanique, p. 1022). 



In Yaucheria, where the structure is in many points remarkably 

 like that of the Peronosporese, no question has ever arisen as to 

 the propriety of regarding as a sporangium the cell, cut off from 

 the rest of a thallus by a transverse wall, in which the zoospore is 

 formed. There is in Yaucheria the most complete series of grada- 

 tions, from the production of ciliated zoospores to spores which 

 have no cilia, and do not leave the sporangium, to complete apos- 

 pory in Vaucheria tuberosa. In this species all the phenomena 

 associated with the formation of a sporangium are observed, but 

 the isolated sporangium developes into a new thallus without 



