610 AQ VATIC PHYCOMYCETES 



quent development of the thallus differs with each genus and even with 

 species of the same genus. 



The peculiar, often pitted, resting spores so characteristic of this order 

 are unquestionably a device for tiding the plant over unfavorable 

 environmental circumstances. Unlike other parts of the fungus they 

 can withstand drying and freezing and probably high temperatures. 

 Although found for the most part on the zoospore-bearing plant they 

 are occasionally observed on the gametophyte, if it is formed. Their 

 structure in practically all instances is identical. In the early stages of 

 development they resemble in position and shape ordinary zoosporangia. 

 The increasing density and darkening of the protoplasm and the thick- 

 ening and pigmentation of the wall, however, together with the lack 

 of formation of discharge papillae, soon distinguish them from devel- 

 oping zoosporangia (Fig. 40 F, p. 616). The brownish pigment charac- 

 teristic of these resting spores has in several species of AUomyces been 

 found by Emerson and Fox (1940) to belong to the melanin group. At 

 maturity the obpyriform, ovoid, clavate, beaked, or sometimes spher- 

 ical resting spore is more or less closely enveloped by the thin wall of 

 the container within which it lies. The outer wall of the spore is thick, 

 of varying shades of brown, and usually perforated by innumerable 

 minute inwardly directed cone-shaped pits or pores. The apices of these 

 pores are in contact with a second, thinner, smooth colorless wall which 

 surrounds the cytoplasm. The living contents, although considerably 

 masked by the pigmented outer wall, are composed of numerous large 

 fat globules imbedded in a finely granular matrix. The resting spores 

 vary somewhat in shape in the different species, but they frequently 

 have a narrow truncate base and, for the most part, are broadest slightly 

 above the equatorial region (Fig. 40 D,F). Although the container or 

 hyphal sheath may burst to liberate its resting spore, more commonly 

 it persists around the resting spore and is shed with it. If persistent, 

 upon germination the container bursts and the thick outer wall of the 

 resting spore cracks open along a preformed line. Discharge papillae 

 develop on the bulging, exposed, thin, inner wall and deliquesce to form 

 pores through which the posteriorly uniflagellate planonts emerge (Fig. 

 42 B, p. 626). In certain forms a somewhat prolonged discharge tube 

 is produced. What type of plant is subsequently developed from these 

 swarmers depends upon the particular organism involved. 



