36 FORMATION OF SPORES OF RHIZOPUS AND PHYCOMYCES. 



and the less dense protoplasms, but, as has been already pointed out, 

 the transition is quite sudden. Still further, the laj^er of dense pro- 

 toplasm is not of the same absolute thickness for all sporangia, nor 

 does it bear a constant relation to the size of the sporangium. The 

 writer has been rather inclined to regard the thickness of this la3^er as 

 dependent on the aniQunt of available protoplasm, though by no means 

 certain on this point. Arthur's conclusions bear more specifically on 

 the streaming in the hypha? than in the sporangium, and yet he gives 

 us no intimation that he wishes to separate the two processes and refer 

 them to different causes. 



In the formation of the oosphere in some of the Peronosporese we 

 have, according to AVager (1896), Stevens (1899), and others, a differ- 

 entiation of the protoplasm into ooplasm and periplasm, but this 

 differentiation is not characterized by such a marked difference in the 

 density of the two protoplasms as in the Mucorinea. The wall about 

 the oosphere is described as forming on the boundary between two 

 protoplasms. The question as to just how the protoplasm is divided 

 and the wall formed has been pretty carefully avoided by all these 

 authors. Stevens states that there is a thin film formed between the 

 two protoplasms, and that this film seems to develop into the wall of 

 the oospore, but his account of the process is very incomplete. Trow 

 (1901) figures a stage in Pythl.um uHhmim, in which the oosphere is 

 only partially cut out, but, unfortunately, he does not describe it suffi- 

 ciently to give us a clear conception of the real nature of the process. 

 The fact that the columella cleft forms just inside the denser plasm, 

 rather than between it and the looser plasm, accords well with the idea 

 that the cleft is formed by cytoplasmic contractions. The layer of 

 denser plasm inside the columella cleft seems to be for the specific 

 purpose of aiding in the cleavage by its contraction, a function that 

 the looser plasm is probably unable to perform. 



SUMMARY. 



The essential processes in the formation of the spores in the sporan- 

 gia of RJdzopm and Phjcomycea may be summarized as follows: 



1. Streaming of the cytoplasm nuclei and vacuoles up the sporangi- 

 ophore and out toward the periphery, forming a dense layer next the 

 sporangium wall and a less dense region in the interior, both containing 



nuclei. 



2. Formation of a layer of comparatively large, round vacuoles in 

 the denser plasm parallel to its inner surface. 



3. Extension of these vacuoles by flattening so that they fuse to 

 form a curved cleft in the denser plasm; and, in the case of Rhizopm^ 

 the cutting upward of a circular surface furrow from the base of the 

 sporangium to meet the cleft formed by these vacuoles, thus cleaving 

 out the columella. 



