^30 The Irish Naturalist. [February, 



These zoospores in the genera Sap7vleg7iia, Leptolegniay 

 Pythiopsis, and Leptornittts are endowed with two periods of 

 movement, hence are called diplanetic. They generally 

 escape with free movement from the sporangium, to settle 

 down, after about fifteen minutes activity, and encyst to 

 globular shape. The period of this encystment may vary, 

 mainly depending on the presence of food in their vicinity, 

 but they soon awaken again to active life, and seek for a 

 favourable nidus on which to germinate. 



In the genera Achlya, Apha7iomyces, and Apodachlya, the 

 zoospores have but one period of movement after their escape, 

 hence are called monoplanetic, and the genera Thraustotheka 

 and Dictyuchtis are practically the same, but in the genus 

 Aplaiies movement is entirely suppressed and the zoospores 

 germinate in situ, without liberation from the sporangium. 



The sexual spores, or oospores, are produced in a distinct 

 vessel called an oogonium, and generally appear in the life- 

 cycle of the plant at a later period than the zoospores. Bj^ 

 their increased power of resistance to unfavourable conditions, 

 and their greatly prolonged vitality, they are evidently in- 

 tended to carry reproduction over periods of danger, such 

 as the cold time of winter, or an interval of starvation. Re- 

 searches on the vitality of these spores^ show them to be 

 extremely torpid, so much so that their germination in most 

 species has not yet been observed, while the few which have 

 been induced to grow sometimes demand as much as 200 days 

 rest before they can be awakened to life. 



The rarer non-sexual chlamydospores are supposed by 

 Humphre}^^ to fill an intermediate want, as the}^ germinate 

 more readily than the oospores, and by their greater resistance 

 can constitute a resting-stage when compared with a zoospo- 

 rangium. They are usually vessels of globular shape, often 

 formed in a series, their position being terminal on the 

 bearing filaments. They are not commonly produced, and 

 usually appear at the later stages of vegetation, when con- 

 ditions may be unfavourable for cultivation owing to other 

 forms of life competing, or to want of nourishment. 



^ De Bary, "Morphologie der Pilze." 



' '-The vSaproleguiaceae of the United States," 1893, 



