76 
MORPHOLOGY OF TISSUES. 
between the cells of one row serves rather for a more rapid movement of the 
contents over greater distances. Laiia/erous 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-^valL' 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 
Hyacintbus orientalis as an example. Figs. 61-64 are transverse sections perpendicular 
to the surface of the leaf; ee \n 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, S, this has just 
