No. 450.] STUDIES ON THE PLANT CELL. 453 



portions of coenocytes, generally divide extensively at repro- 

 ductive periods and always through cleavage by constriction with, 

 however, the frequent cooperation of vacuoles which heip~to cut 

 the protoplasm in the same manner as the cleavage furrows. 

 Cleavage by constriction is undoubtedly the most primitive type ; 

 free cell formation and cleavage by cell plates being special and 

 very highly developed protoplasmic activities. 



i . Cleavage by Constriction. 



A simple example of cleavage by constriction is presented by 

 such an alga as Cladophora. The process consists in the build- 

 ing out of a ring of cellulose from the side wall into the cell 

 cavity. The outer plasma membrane forms a fold, thus placing 

 the two surfaces opposite one another (see Fig. 8 a), and the 

 wall is laid down between these. Spirogyra forms its wall in 

 precisely the same manner as Cladophora with this peculiarity, 

 that the new wall finally cuts through the protoplasmic strands 

 that connect the daughter nuclei. These strands are said to con- 

 tain spindle fibers (Van Wisselingh, : 02) which may contribute to 

 the plasma membranes forming the cell wall, as it is completed. 

 Another illustration of cleavage by constriction is presented in the 

 formation of gametes of moulds (Sporodinia) and the abstric- 

 tion of conidia (Erysipheae), both processes having been studied 

 by Harper, '99, p. 506. In these cases a cleavage furrow pro- 

 ceeds from the surface inward and divides the protoplasm. The 

 partition wall of cellulose is formed later between the two free 

 plasma surfaces. The only differences between the processes 

 above described are that in the first forms the cleavage proceeds 

 more slowly and the wall follows the furrow as it progresses in 

 the interior of the cell, while in the latter types cleavage is com- 

 plete before the plasma membranes develop the wall. Cell 

 division in the red Algae (Rhodophyceae) is also a process of 

 constriction similar to Cladophora, but the wall is not generally 

 formed entirely across the filament so that adjacent cells remain 

 connected by thick strands of protoplasm. 



These processes become much more complicated when large 

 masses of multinucleate protoplasm are divided up into many 



