10 ELEMENTARY MEMBRANE PROTOPLASM. [BOOK i. 



membrane, but the separation of a stratum of fluidity as every 

 bubble shows. The half fluid substances, mucus, jelly, &c., 

 are a mixture of solid and fluid parts, as can be seen when 

 they are dried." 



Membrane varies in its degree of transparency, being 

 occasionally so exceedingly thin as to be scarcely discover- 

 able, except by the little particles that stick to it, or by its 

 refraction of light, but in ferns, some fuci, and other cryp- 

 togamic plants, it is brown from its first birth : according 

 to Roper it is green in Viscum album : Link says it is green 

 in the leaves of Ruellia Sabiniana and the petiole of Cycas 

 revoluta ; and Me^en mentions its being orange-coloured in 

 the petiole of many tropical Orchids. 



It is always excessively thin when first . generated ; arid 

 whatever thickness it afterwards acquires must be supposed 

 to be owing to the incorporation or incrustation of secreted 

 matter. This was first observed by Mohl in Palm-trees, 

 where he found a successive addition of strata to the lining 

 of the cavities of the cells ; and is apparently an universal 

 occurrence where membrane becomes thickened. But the 

 matter added to membrane is often so homogenous as to 

 offer no trace of its being deposited concentrically, even 

 when examined by the most powerful microscopes, and I am 

 not always able to discover the regular lines upon its section 

 which are represented so uniformly by the German anato- 

 mists. It is, however, plain enough that the membrane of 



the woody tubes of the liber 

 is in many plants thickened 

 successively by the deposit 

 inside of concentric layers 

 of sedimentary matter, as 

 may be seen in Castanea 

 vesca (fig. 1. a), and Betula 

 alba, and in the cells below 

 the stomates of Pinus sylvestris (fig. 1. b) and there are 

 sufficient traces of it to be found elsewhere to justify the 

 opinion that it is a common mode of increment in thickness. 

 The first layer of matter is invariably soft and azotised, and 

 now bears the well-contrived name of protoplasm, proposed 



