July 15, 191S 



Rheo Sporangium Aphanidermatus 



285 



spores to mature succeed in rupturing the sporangium wall before the 

 others have become mature. Such attached spores are often seen 

 struggling about in the water until the completion of cleavage releases 

 them from each other and they swim away. As already noted, cleavage 

 may occur within the sporangium without its escape from the wall of the 

 presporangium. In such cases the sporangium wall draws away from 

 that of the presporangium so as to make it distinctly visible (Pi. XLIV, 

 fig. I). 



The mature zoospores are uninucleate; hence, the number to develop 

 in a given sporangium is predetermined by the number of nuclei con- 

 tained within the presporangium. The number A^aries within wide limits; 

 but, so far as accurate counts have been made, it has always been even. 

 The zoospore nucleus is distinctly top-shaped, with the nucleolus at the 

 broad end. The pointed end is directed toward the flattened side of the 

 spore, where it ends in a blepharoplast, from which the cilia arise (Pi. 

 XLV, fig. 3, and XLVI, fig. 6, 7, 9, 10, 11). The spores are plano- 

 convex or slightly concavo-convex, with a sinus in the flattened side, 

 which in the uncleaved sporangium is directed outward. The blepharo- 

 plast appears to come in contact with the spore membrane at the base of 

 this sinus (PI. XLVI, fig. 9, 10, 11). 



It has already been pointed out that the cilia become visible in the 

 living material before cleavage is complete. The sectioned material 

 bears out this observation (PI. XLV, fig. 3). When first observed in 

 hanging-drop preparations, the cilia are quite short; but it may be seen 

 that they elongate rapidly during the progress of cleavage so as to give 

 the appearance of being pushed out slowly from within. The growth is 

 sufficiently rapid almost to be seen at magnifications of 2,000 diameters — 

 that is, one readily notes that they have increased in length in the course 

 of a few seconds during which the attention has been fixed upon them. 

 When mature, they are relatively long, but of unequal length. It is 

 interesting to note that their combined length approximates twice the 

 greater circumference of the spore, as is shown in Table I. 



Table I. — Relation of length of the cilia to the circumfere-nce of the spores (in microns) 



