GENERAL CONSIDERATIONS. 35 



secretion of the rytophisiu through the vticuohir inenibnine, and the 

 fact that the substance is secreted only in those vacuoles which are to 

 take part in the cleavage seems to indicate a difference, in function at 

 least, between the membranes of the two kinds of vacuoles. 



The clear zone between the bodj'^ inside the vacuole and the vacuolar 

 membrane seems to be due to the contraction of the substance in 

 dehydration. 



The fact that this substance takes so readily the shape of the vacuole 

 or the furrow that contains it would show that in the living state it is 

 not solid, but very plastic, if not in actual solution. 



Stevens (1899) describes a gelatinous, stainable substance in the vacu- 

 oles of the oogonium oiAlhxujo hlit'i^ and seems to regard it as being used 

 to form the walls of the oospore. He says of it: "It appears to be trans- 

 ferred directlv from the vacuoles to the exteriorof the protoplasm, there 

 to be chano-ed to true cellulose." Whether or not this substance is the 

 same as that in Phycomyo'^ I can not be certain. Stevens's description 

 agrees very well with my own in that the substance takes the stain 

 only slightly when first formed, and stronger in later stages. In 

 Albugo^ however, it leaves the vacuoles and goes to form cellulose 

 walls, while in Phycomyces it never disappears, but forms the inter- 

 sporal substance in the clefts made by the vacuoles and apparently 

 pla3's no part in the formation of walls. Stevens describes this sub- 

 stance as occurring in figs. 91, 92, 93, and 9-1: of his PI. XV, 1)ut I 

 have been unable to find any representation of it in the places referred 

 to. Neither does he describe the method by which it is transferred to 

 the periphery of the oospore. 



Trow (1901) also has figured a similar content in the vacuoles of 

 Pythium ulthnum, but does not describe it so as to give any idea of 

 its true nature. 



A problem that has been most perplexing to me is how the proto- 

 plasm in the sporangium comes to be differentiated into a very dense 

 layer at the periphery containing many nuclei, and a very loose struc- 

 ture in the interior with few nuclei. The purpose of such a differen- 

 tiation is very evident, viz: That as much of the protoplasm as 

 possible may be included within the spores, but just what propels the 

 protoplasm up the sporangiophore and out toward the peripher}- of 

 the sporangium is not so easy to determine. Arthur's (1897) explana- 

 tion that it is due to evaporation of water from the surface, combined 

 with absorption of moisture from the substratum, seems entirely inade- 

 quate. If this were the cause, we should expect to find the layer of 

 denser protoplasm at the base of the sporangium as well as on the 

 sides and top, as we have no evidence that evaporation does not go on 

 from the part of the sporangium just around the sporangiophore as 

 well as from the rest of the surface. Furthermore, from Arthur's 

 explanation we should expect a gradual transition between the denser 



