No. 4SO.] STUDIES ON THE PLANT CELL. 455 



mined entirely by the activity of vacuoles. Spore formation, 

 however, is accomplished by cleavage furrows which progress 

 from the exterior inwards and, without the aid of conspicuous 

 vacuoles, cut out multinucleate masses of protoplasm which 

 become the spores. 



Dean Swingle (: 03) has extended the studies of Harper on 

 spore formation in the molds to Rhizopus and Phycomyces. He 

 confirms Harper's account of the general processes of cleavage 

 by furrows cooperating with vacuoles, and notes the following 

 characteristics in the types studied. In Rhizopus the position 

 of the columella is determined by a dome-shaped series of flat- 

 tened vacuoles which fuse and meet a cleft that extends upward 

 from the outer plasma membrane at the base of the sporangium. 

 The spores are formed in Rhizopus by branching systems of 

 curved furrows that cut the protoplasm into multinucleate masses, 

 and in Phycomyces by angular vacuoles that develop into furrows 

 which extend in various directions and unite with one another 

 and with clefts from the region of the columella. 



Other excellent illustrations of cleavage by constriction are 

 presented in the sporangia of such types as Hydrodictyon, Clado- 

 phora and Saprolegnia. Timberlake (: 02) has given an account 

 of Hydrodictyon, and the events are also fairly well understood 

 for Saprolegnia. Segmentation begins in Hydrodictyon by the 

 development of cleavage furrows in the outer plasma membrane, 

 which, cut into the protoplasmic layer at right angles to the 

 surface and meet similar furrows that make their way from the 

 large central vacuole outward. These cleavage planes spread lat- 

 erally, uniting with one another, until the protoplasm is all 

 divided into uninucleate masses which become the zoospores 

 (Fig. 8 f). In Saprolegnia (see Davis, : 03, for general account) 

 conspicuous cleavage furrows develop from the central vacuole 

 and make their way to the exterior, finally breaking through the 

 outer plasma membrane. When this takes place there is an 

 immediate escape of cell sap, which was under pressure, and a 

 shrinkage of the sporangium so that the zoospore origins appear 

 to fuse, but this is not really the case, for cleavage is continued 

 and the zo5spores soon separate. 



A physiological explanation of cleavage by constriction must 



