RUJUVENESCENCE IN NATURE. 1*77 



bursting, and sometimes through chemical softening and 

 solution. The cases of forcible tearing of the cell-mem- 

 brane are themselves of varied kinds ; we distinguish: 1, 

 tearing of the cell-membrane, without its being* wholly 

 peeled off; 2, complete peeling of the cell-membrane, 

 skinning of the cell ; and, 3, slipping-oul of the rejuvenised 

 cell -structure from the old membrane. 



1. One of the most beautiful examples of the tearing 

 of an outer cell-membrane, from its not keeping pace 

 with the growth of the inner cell, has been described 

 by Schiinper* in the paraphyses of Diphyscium foliosum. 

 These paraphyses are composed of a simple row of cells, 

 which become very considerably elongated during the 

 completion of their development. As soon as they have 

 attained about half their length, the outer cell-membrane 

 splits annularly in the middle of the cell, like a circum- 

 scissile capsule ; the two halves separate and remain 

 attached upon the links as bell-shaped or bowl -shaped 

 collars, while the cells continue their longitudinal growth, 

 sometimes opening and retracting their walls in the same 

 manner a second time. I have observed in certain 

 Confervae, especially Ulothrix Braunii,\ and Zygogonium 

 ericetorum, conditions comparable with the process in 

 Diphyscium. The delicate filament of the first-named 

 plant is composed of a row of cells, which divide by 

 double horizontal division, each into four cells. The 

 membrane of the mother-cell of this group of four cells 

 tears crossways, during this process of division, into two 

 equal halves, which remain as short, abrupt sheaths on 

 the upper and lower ends of each four-celled group. 

 Zygogonium ericetorum exhibits the same phenomenon in 

 manifold repetitions, and even in cells which have ceased 

 to divide, and are only undergoing elongation ; but the 



* W. P. Schimper, Recherches sur les Mousses,' (1848,) p. 52, pi. vi, 

 f. 42-46. 



f Kiitzing, ' Spec. Alg.,' p. 346. The above-mentioned species can 

 scarcely Jae included in the same genus with Ulothrix zonata, since it forms 

 only two germ-cells in a mother-cell, which, moreover, possess not four, but 

 only two cilia, as in the aquatic Hormidia. 



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