74 MORPHOLOGY OF TISSUES. 



contents of the cells is not replaced by air ; the communication established between 

 the cells of one row serves rather for a quicker movement of the succulent contents 

 over greater distances. The Laticiferous Vessels must also be regarded as com- 

 posed of coalesced cells ; they are the result of very early and complete absorption 

 of the 'partition-walls of adjoining cells belonging to straight or much branched rows 

 in the interior of different systems of tissues. 



Here however tubes produced by the coalescence of cells need only be opposed 

 to intercellular spaces for the sake of contrast ; a more miiuite consideration will 

 come better in connexion with the description of the systems of tissues. 



(a) ' Intercellular Substance'' and '■Primary Cell-^all.^ The hypotheses implied by 

 these terms could only be entertained so long as it was supposed that the original 

 thin lamellae between two adjoining tissue-cells were double ; and so long as it was be- 

 lieved that the stratification of the cell-wall was brought about by apposition of new 

 layers. The expression that the original thin partition-wall between two tissue-cells is a 

 double lamella can only be understood in two senses ; either it means that the lamella 

 consists of molecular layers, and that two of these contain between them the ideal 

 boundary-surface of both lamellae belonging to the adjoining cells, or that there is 

 an actual interruption of the molecular connexion, and from the first an actual cre- 

 vice. The last supposition is inadmissible, since it does not rest upon observation ; 

 it is besides contradicted by the detection of weak boundary lines between layers which 

 nevertheless are molecularly united, and have no crevices between them. Thus in the 

 layers of thick cell-walls and of starch-grains there are no crevices, and yet the bounds 

 of the layers may be seen ; why then should the assumed actual crevice not be visible 

 in the original partition-wall? If now the first alternative is assumed to be correct, and 

 the composition from two lamellae considered as purely ideal, the question depends 

 on a mere verbal controversy with reference to the intercellular substance ; for if the 

 original homogeneous partition-wall, although consisting of molecular layers, is yet held 

 together everywhere by molecular forces, and the supposed boundary surface is no 

 interruption of the molecular structure, then the deposition of a different substance 

 (intercellular substance) at the same place appears only as a process of ordinary growth 

 by intussusception. The fact that the boundary line between previously separated cells 

 disappears by subsequent coalescence proves that the outer molecular layers of cell- 

 walls already formed may yet enter into molecular union. If in such cases a differen- 

 tiated middle lamella is afterwards formed, this is the most striking evidence against 

 the explanation of it as primary cell-wall. If an attempt is made further to construe 

 on paper step by step the behaviour of a developing woody tissue, for example, while 

 retaining the theory of the primary cell-wall, one is immediately involved in difficulties 

 which do not arise on the supposition that the middle lamella is simply the result of 

 subsequent differentiation of the cell-wall. 



(b) Addition to the Intercellular Spaces. With the origin of these spaces is very often 

 connected, as has been mentioned, a peculiar development of separating cells, quite 

 different from that of the rest of the tissue ; so that the intercellular space, together 

 with its environment, represents, in a certain sense, a peculiar form of tissue or an 

 organ for a definite purpose. The observation of some cases of this kind is well calcu- 

 lated to show the beginner how, even in the domain of tissue-formation, morpholo- 

 gically similar or equivalent processes lead to entirely different results from a physio- 

 logical point of view. This subject will be treated in a more general and detailed 

 manner in the third chapter, and in Book III. 



(i) The cleft of the stomata of the epidermis belongs also to the category of 

 Intercellular Spaces, and its origin is peculiarly calculated to afford an insight into the 

 mode of formation of an intercellular space. I have chosen the stomata on the leaves of 



