108 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 miotion 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 
1% 


FIG. 53. Phytophthora infestans, Mont. @ a sporangium lying FiG. 54. Cladochytrium Iridis. a resting- 
in water after the division is complete. 4 escape of the 10 swarm- spore with a brown membrane seen from the broad 
spores from the sporangium. c sporesin the motile state. @ spores side. 4 the same rotated through 90° in the centre is 
at rest and beginning to germinate. Magn. 390 times. a large fatty spherical body. c—e successive stages 
of, ination ofa single sp ; the brown outer 

coat opens by a lid and the inner cell developes into 
a tubular receptacle of swarm-spores. @ formation 
of the spores completed. e escape of the spores. 
fa single swarm-spore. a—e¢ magn. 375 times, / 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-shaped 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 
