FORMS AND SYSTEMS OF TISSUES. 
«3 
primary and of the various forms of secondary meristem which no longer divide, but 
^Yow vigorously, and finally attain a definite development, in which they are of service 
to the plant by the rigidity or some other property of their cell-walls, or by the 
chemical activity of their contents. The most various forms of permanent tissue result 
from similar cells of the same primary or secondary meristem. 
On attaining their permanent condition many forms of permanent tissue lose the 
whole of their living contents, especially their protoplasm, of which dry granular vestiges 
are sometimes left behind. Such forms of tissue may be termed Kenenchyma^, and 
include, for example, cork-tissue, and the tracheal elements of the wood-vessels and 
vessel-like wood-cells. In contrast to this is the Succulent Tissue, the cells of which, 
during the life of the organ, remain filled with chemical products of the vital activity 
of the plant, not with air or water. These succulent tissues may again be divided 
into two groups: — In the first the cells still contain protoplasm in an active vital 
condition, and are therefore able, under favourable conditions, to grow and divide 
afresh — i.e. to pass over into older meristem, phellogen, &c., or, on being wounded 
to form a callus or cork-tissue — as, for example, in chlorophyll-containing tissue, 
the succulent parenchyma of cortex, or of tubers, &c. In the second kind the cells 
pass into a condition of permanent quiescence, as in all those cases in which the 
protoplasm becomes unrecognisable, or remains behind as a doubtful residue ; and 
where the cells are filled, not with chlorophyll, starch, sugar, inulin, fatty oil, aleurone, 
or other reserve-materials, but with excrementitious products of various kinds, as 
volatile oils, resin, gum, cystoliths, clusters of crystals, &c. Cells of this description, 
and tissues composed of them, are apparently never capable of any further develop- 
ment ; they are incapable, for example, of forming a healing cork-tissue over wounded 
surfaces. There is however no sharp line of demarcation between these different 
forms of tissue distinguished by their contents ; it is only the extrem.e cases that can 
be thus characterised. 
We also find the greatest variety of intermediate links, if we consider the various 
kinds of tissue in reference to the thickness and consistency of the cell-wall. 
Starting from ordinary succulent parenchyma with thin but firm and elastic walls 
composed of nearly pure cellulose, we see how, on the one hand, the thin cell-walls 
become converted into cork (periderm), while on the other hand the cell-walls of 
other kinds of tissue thicken and become lignified or converted into mucilage, or 
as hard as stone. The nature of lignification and conversion into mucilage has 
already been pointed out; the coUenchymatous development of certain hypodermal 
tissues will be spoken of hereafter ; here it is necessary only to refer to the fact 
that layers, strings, or groups of cells are frequently distinguished by the extraordinary 
hardness and thickness of their cell-walls. Such tissues, which may arise in all systems 
■ — as for instance the ' Stone-cells ' (scleroblasts) in the flesh of pears and in the bark 
of many trees, the dark-brown strings in the stem of Tree-ferns, &c. — may be in- 
cluded under the collective term Sclerenchyma. 
If we now consider tissues in reference to the form and the mode of combination 
of their cells, it is of course evident that the latter must depend on the former ; but 
that, on the other hand, the position of cells already existing must influence the 
growth, and therefore the form, of those which are developing. If we for the moment 
leave idioblasts out of account, and consider merely the mode of association of similar 
cells, we find that the old distinction into Prosenchyma and Parenchyma can still be 
maintained. By the former is meant a grouping together of elongated fusiform pointed 
and usually thick-walled cells, whose ends are dove-tailed between one another without 
intercellular spaces. A prosenchymatous arrangement of this kind is well seen in 
^ [This term, which has not hitherto been employed, is proposed as the equivalent of the 
German ' Leerzellengewebe.'] 
