REVIEWS. 413 



changed to cuticles, slimy sheaths, t&c. ; possibly the 

 alteration in structure is connected with the tensions they 

 have to undergo, as well as the action of the environment. 



The tearing away of outer layers in Microspora, and 

 the formation of intercellular substance in some Algse, as 

 well as of caps, &c., support these views. 



Schmitz leaves undecided whether intussusception does 

 or does not aid in these processes. Strasburger does not 

 admit this, and considers it only possible, not probable, 

 that protoplasm, penetrating between the cellulose particles 

 (which have arisen as disintegration products of protoplasm), 

 there deposits new microsomes where required. 



The professor then proceeds to some observations of con- 

 siderable interest. In the bark of some Conifers the sieve- 

 tubes are pulled tangentially by the enlargement of the 

 cambium, and are pressed by the cortex and cork until the 

 lumen is obliterated, and a horny, single, inseparable layer 

 represents the tube ; in the wood the young radial walls of 

 the cambial cells become thinner as they are stretched, and, 

 although several lamellae appeared to have been deposited, 

 at length form a single, thin, homogeneous membrane. In 

 other words, by pressure and tension several lamellae may 

 become fused into a single homogeneous layer. 



As in such cases as the passage of zoospores through cell- 

 membranes, the in-boring of parasitic filaments, &c., it may 

 be supposed that protoplasm can soften already dense cell- 

 walls ; and Strasburger believes that some such process 

 occurs in the branching of CladopJiora, where the wall 

 bulges at a certain point, and the before lamellated mem- 

 brane becomes stretched to a thin, homogeneous one. In 

 unicellular plants, where hydrostatic pressure alone cannot 

 cause curvatures, the curving produced by light or gravi- 

 tation may be accompanied by softening and stretching of 

 the membrane at certain spots. Weisner says that in posi- 

 tively heliotropic , plants the elasticity of the illuminated 

 side is increased, so that the stretching on the shaded side 

 is greater. 



Other examples are quoted, as Ulothrix, and then some 

 observations on Spirogyra. The cell-wall here only presents 

 two layers, an outer one which may become slimy, and an 

 inner, more refractive one, of the nature of a "limiting 

 membrane; " a dark line separates the two. It is interesting 

 to note that in aS". orthospira the separation of the cells 

 takes place so suddenly and sharply that they may fly apart 

 to some distance. 



The free end of such a separated cell soon shows the in- 



