76 MORPHOLOGY OF TISSUES. 



between the cells of one row serves rather for a more rapid movement of the 

 contents over greater distances. Laticiferous Vessels must also be regarded as 

 composed 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 different systems of tissues. 



Here however it is only necessary to point out the contrast between vessels 

 produced by the coalescence of cells and intercellular spaces; a more minute 

 consideration will come better in describing the systems of tissue. 



(a) ^Intercellular Substance' and ^Primary Cell-wall.' The hypothesis implied by 

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

 thin lamella between two adjoining tissue-cells was double, and so long as it was 

 believed that the stratification of the cell-wall was brought about by the deposition of 

 new layers. The expression that the original partition -wall between two tissue-cells 

 is double 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 the two adjoining cells, or that there is an actual interruption 

 of molecular connexion. The last supposition 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 cleft between them. 

 Thus in the layers of thick cell-walls and of starch-grains there are no clefts, and 

 yet the boundary-lines between the layers may be seen. If the first alternative is 

 assumed to be correct, the question with reference to the intercellular substance 

 depends on a mere verbal controversy ; for if the original homogenous partition-wall 

 is held together everywhere by molecular forces, and the supposed boundary-surface 

 is no interruption of the molecular structure, then the deposition of a special inter- 

 cellular substance at the same place is nothing but a process of ordinary growth by 

 intussusception. The fact that the boundary-line between cells previously separate 

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

 walls may enter into molecular union. If in such cases a middle lamella is after- 

 wards differentiated, this is the most striking evidence against the explanation of 

 it as primary cell-wall. If an attempt is made to follow step by step the be- 

 haviour of developing woody tissue on 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. 



(b) Examples of the formation of Intercellular Spaces. The origin of these spaces 

 is very often connected, as has been mentioned, with a peculiar development of 

 the separating cells, quite different from that of the rest of the tissue ; so that the 

 intercellular space together with its surrounding cells constitutes a peculiar form of 

 tissue or an organ for a definite purpose. The observation of some cases of this 

 kind is well calculated to show the beginner how, even in tissue-formation, processes 

 which are morphologically similar or equivalent lead to entirely different physiological 

 results. 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 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 

 Hyacinthus orientalis as an example. Figs. 61-64 are transverse sections perpendicular 

 to the surface of the leaf; ^^ in all of them are the epidermal cells, />/> the parenchyma 

 of the leaf. The stoma, S, is formed of a rather small epidermal cell, which divides 

 into two equal sister-cells by a wall vertical to the leaf; in Fig. 61, 5, this has just 



