30 FORMATION OF SPORES OF RHIZOPUS AND PHYCOMYCES. 



has attempted to demonstrate diagrammatical ly the way in which 

 such localized contractions would cut up the protoplasm in exactly the 

 same manner actually occurring in these sporangia. The type of 

 cleavage represented in l*Uoholus has been chosen so that the same 

 diagrams may ])e used to explain vacuolai- and surface-furrow cleavage. 

 For the sake of clearness the diagrams were made much simpler than 

 the actual sporangia, but without changing any essential fact of struc- 

 ture. The lines of force caused by the contraction of the cytoplasm 

 have been represented by arrows — red indicating a localitv in maxi- 

 mum contraction; green, a locality that has not yet reached its 

 maximum: and blue, a locality that has passed its maximum. Where 

 there are wide spaces between arrows there is assumed to be little or 

 no contraction. Dotted black lines represent planes where cleavage 

 will take place. 



For the cutting out of the columella, let us assume that after the 

 system of vacuoles shown in PL VI, fig. 28, is formed, the cytoplasm 

 at such points between l)ut close to the Aacuoles, as shown by the red 

 arrows, begins to contract in a direction at right angles to the future 

 columella cleft, the spore-plasm pulling toward the periphery and the 

 columella-plasm toward the center of the sporangium. This would 

 tend to pull the cytoplasm away from the points at the rear ends of 

 the arrows and also to draw the general masses of the spore-plasm 

 and the columella-plasm toward each other. This would cause pres- 

 sure ao-ainst the sides of the vacuoles and cause them to flatten out to 

 fill the spaces from which cytoplasm is being withdrawn, as is shown 

 in PI. Yl, fig. 29. At the l)ase of the sporangium where the surface 

 furrow is to cut in, the cytoplasm contracts in a direction approxi- 

 mately radial to the curve in which the furrow is to cut. This pull- 

 ing causes a rift beginning at the surface of the sporangium, and 

 the viscid plasma-membrane, ever adhering to the surface of the cyto- 

 plasm, folds in to line this rift. As the furrow cuts inward, the 

 points of greatest contraction move inward also (PI. \1, fig. 29), keep- 

 in o- alwavs close in front of the furrow until the latter fuses with the 

 lowest vacuoles in the system. 



The principle involved in the cleavage of the spore-plasm is essen- 

 tially the same. At the points indicated by red arrows in PI. VI, 

 fig. 30. viz, on the periphery, on the columella cleft, and on the vacu- 

 oles, the cytoplasm begins to contract in a direction approximately 

 toward the centers of the masses of protoplasm that are to become the 

 spores. This pulling away of the protoplasm causes rifts or furrows 

 running into the spore-plasm from the periphery, the columella cleft, 

 and the vacuoles, as shown in PI. VI, fig. 31. The width of the fur- 

 rows depends on the continuation of the contraction after the furrows 

 have progressed beyond the points of contraction, i. e.. on the amount 

 of contraction that takes place at the points marked in the diagrams 



