R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 1G7 



bar is projected inward ; the infolding of the primordial membrane 

 is instantly followed by division of the cellulose case. 



To study the division of the cell nucleus, which in Cladophora 

 is concealed by the other contents of the cell, we have better 

 material in Spirogyra, Zygnema, and Ulothrix, in which division 

 begins with that of the nucleus of the mother-cell, and as soon as 

 this is comj)leted, infolding of the primordial membrane com- 

 mences. 



In Q^dogonium, if we examine a mother-cell before it attains 

 division, we find the upper of two adjacent cells appears enclosed 

 by two cellulose membranes ; the inner, more strongly developed, 

 forms the cellulose case of the cell under observation ; the outer, 

 extremely thin, belongs, on the contrary, to the preceding genera- 

 tion, i.e., the mother-cell, in which the two cells before us have 

 been formed. In the lower of the two cells, yet a third coat is ap- 

 parent, which, close under the upper end joining the first cell, is cut 

 off by a distinct transverse line, indicating the place of previous 

 division. The commencement of the process is seen in an infold- 

 ing of the inner cellulose case just under the youngest cap, so that 

 the cell contents appear drawn in by a ring, the space between the 

 fold and the outer case being empty. During the formation of this 

 ring-fold, the nucleus lying in the upper half of the mother-cell 

 enlarges to double its original size, and then divides into two 

 nuclei — one for each daughter-cell — and at the same time on a 

 level with the line of division of the nucleus appears a ring-like 

 collection of colourless protoplasm, immediately followed by an in- 

 folding of the primordial membrane, which divides the mother-cell 

 into two daughter-cells, the upper of which is the shortest. But 

 before the lacing-in of the membrane is completed, the outer case 

 splits gradually, just above the infolding of the cellulose case, 

 Avhich then becomes stretched out, so that the fold disap- 

 pears. 



With the division of the young cellulose case, the lower 

 daughter-cell elongates and pushes the upper one before it, while 

 the cellulose case of the mother-cell becomes more and more elon- 

 gated. As soon as the lower daughter-cell has grown up to the 

 transverse stria of the sheath, produced by the splitting of the 

 outer case, the division appears, i.e., the unlacing is completed, and 

 the young cellulose case has developed over the primordial mem- 

 brane, so that now two complete daughter-cells are present. Now 



