FORMS AND SVSTEMS OF TISSUES. Set 



primary and of the various forms of secondary meristem which no longer divide, but 

 grow 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 diff'erent 

 forms of tissue distinguished by their contents ; it is only the extreme 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 collenchymatous 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 jnany 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.'] 



