jo8 



DIVISION I. GENERAL MORPHOLOGY. 



posterior being directed backwards and dragged behind. In Phytophthora infestans, 

 according to older observations which perhaps need revision, both cilia spring from 

 a point on the posterior margin of the pellucid spot. 



These ' beanshaped' zoospores are peculiar to the Peronosporeae and Saprolegnieae. 

 Similar but not the same forms are found in certain Chytridieae (see section XLVI). In 

 the genera Achlya, Achlyogeton, and Aphanomyces the spores are discharged from 

 an orifice in the sporangium without cilia and autonomous movement, and collect 

 before the orifice in close lateral union with one another, forming the surface of a 

 hollow sphere and a small head. As they pass from the sporangium on to the 

 surface of the sphere, each of them becomes itself spherical and forms a thin 

 firm cellulose-membrane. After a rest of some hours the protoplasm escapes from 

 this cell-wall and then assumes the characters of the bean-shaped swarm-spore. It 

 is only after the escape of the protoplasm that the cilia slowly develope at the spots 

 which have been described, and as they develope the movement sets in, beginning 

 as a slight swaying motion and passing by degrees into quicker rotation and finally 

 into rapid movement of translation. In the sporangia of most species of the genus 

 Dictyuchus, as described in section XVIII, the spores are separated by firmly united 

 cellulose-septa ; after some hours' rest the protoplasm issues from the cellulose which 



FIG. 53. Phytophthora tnfutans, Mont, a a sporangium lying FIG. 54. Cladochytriitm Iridis. a resting- 



in water after the division is complete, b escape of the jo swarm- spore with a brown membrane seen from the broad 



spores from the sporangium. <r spores in the motile state, rf spores side. * the same rotated through 90, in the centre is 



at rest and beginning to germinate. Magn. 390 times. ^e e fatty spherical body, c-e successive stages 



of germination of a single specimen; the brown outer 

 coat opens by a lid and the inner cell developes into 

 a tubular receptacle of swarm-spores, d formation 

 of the spores completed, e escape of the spores, 

 /"a single swarm-spore, a f magn. 375 times, f 600 

 times. 



surrounds it and then becomes a swarm-spore as in Achlya. But the spores are not 

 discharged through a single orifice in the sporangium, but each pierces through the 

 nearest spot of the lateral wall of the sporangium, and even also through the empty 

 membranes of the adjacent sister-spores. 



In the genus Saprolegnia a peculiar mode of proceeding is the rule. The spores 

 are discharged from the orifice of the sporangium as ovoid motile bodies with 

 the pointed extremity anterior in the swarming. This extremity is hyaline and 

 two cilia project from its extreme point. The broad posterior portion is formed 

 of granular protoplasm in which three small hyaline vacuoles lie immediately 

 beneath the surface at laterally equidistant spots in the same transverse section. The 

 spore comes to rest after a brief period of movement which does not last usually 

 longer than a few minutes, assumes a spherical form and becomes invested with a 

 thin cellulose-membrane ; but some time after, some hours or even days, it emerges 

 again from the state of rest, the protoplasm escapes as in Achlya out of the membrane 

 and is transformed into a bean-shapqd swarm-spore. Individual exceptions to this 

 dimorphism of single spores occur in all species, in so far as a spore may omit the 

 second swarming-period and pass directly from the first state of rest to germination. 



The majority of the Chytridieae have small round swarm-spores which are capable 

 of motion as they leave the sporangium (Fig. 54). Their protoplasm, which is 



