438 Prof. Meyen's Report of the Progress of 



tions; never, on the contrary, between simple sacs in any per- 

 ceptible quantities ; the cause of which lies in the very exist- 

 ence of the intercellular substance. Valentin's observations 

 also show, that the intercellular substance does not exist at 

 the beginning, but occurs first after the commencement of lig- 

 nification, and must therefore certainly be distinguished from 

 the substance which is found around and between the sacs of 

 the lower Cryptogamia. We have however previously men- 

 tioned, that this substance is also wanting in germinating 

 Conferva?. Since the intercellular substance, observes Va- 

 lentin, appears first after the process of lignification*, it can- 

 not be regarded as an organic mucus which holds the cells 

 together. It is no less a secondary layer without the primi- 

 tive sac than the ligneous lamellae are within the same. It 

 occurs only where a considerable number of ligneous lamellae 

 exist. If we may use an old technical expression, the intercel- 

 lular substance occurs in all cases where thick-sided cells are 

 closely connected with one another, few or no intercellular 

 passages remaining. 



Closely connected with this subject are the observations 

 which have been made on the structure of cellular membrane. 

 Mohl in the above-mentioned memoir has given an accurate de- 

 scription of the stripy structure of the partitions of the cells of the 

 liber of Nerium Oleander, Vincaminor, and various other plants 

 belonging to the families Apocynece and Asclepiadece. These 

 cells show in a diagonal section, as also in a horizontal one, 

 that their sides consist of a great number of superincumbent 

 membranes. The cells of the liber of Vinca throw more light on 

 the subject; Mohl describes them as broad, abruptly and 

 greatly contracted at their ends, sides not very thick, and com- 

 posed of several layers. In the wider parts the membrane was 

 covered with spiral, steep, ascending lines, and so that a part of 

 these lines wound round to the right, the other to the left, and 

 in this manner the membrane was divided into small rhorn- 

 boidal portions. Mohl previously supposes, that the lines are 

 wound in the one layer to the right, in the other to the left, 

 and that the layers which compose these cellular membranes 

 are not homogeneous, but possess a fibrous texture. 



" Are we now to deduce from this aspect of the tubes of the 

 liber of the plants above cited," says Mohl, "the view already 

 entertained by Grew, that the cellular membrane is woven toge- 

 ther of fibres ? I do not think so. As far as we can observe of 

 these extremely delicate formations, discernible only with good 



* The author, differing from other phytotomists, understands by ligni- 

 fication nothing more than the thickening of the sides of the cells by the 

 deposition of new layers. 



