8a MORPHOLOGY OF TISSUES. 



diflerent from that of the surrounding ones, owing to a difference either in its form 

 or its contents; as for instance the stellate hair formed in the interior of the tissue 

 of NymphaeacejE (Fig. 68), and the Lithocysts in which the cystoliths (Fig. 53, P- ^^) 

 are formed, &c. Cells marked by such a striking peculiarity I propose to call 

 Jdioblasts; the special forms will be described further on. I'he term Eremoblast, on 

 the other hand, may be given to all those forms of cells which detach themselves 

 altogether from their sister-cells, and continue an independent life ; such as the spores 

 of Cryptogams, antherozoids, pollen-grains, some gemmae of Hepaticae, &c. The entire 

 individuals of some lower plants are eremoblasts, as most Desmidieae, Bacillarieae, 

 and Siphoneae {Unicellular Plants). There is this in common between idioblasts and 

 eremoblasts, that the individual cells usually attain a high degree of development; but 

 with this difference, that in the former the development is commonly in one direction 

 only, while in the latter the individual cell may undergo the most varied differentiation, ' 

 constituting in itself an organism endowed with independent life. 



(b) Nageli classifies all the various forms of tissue under two primary heads : — 

 Generating Tissue or Meristem, and Permanent Tissue. This classification is by no means 

 parallel wdth one into younger and older cells ; for, even when the former consists of 

 younger, the latter of older cells, there are also other additional and important 

 differences. 



(i) Meristem consists of cells which increase slowly in size, and divide repeatedly 

 in such a manner that some of the resulting cells continue to divide, while others 

 pass over into permanent tissue. The cells which are still capable of division 

 (merismatic) are mostly much smaller than the permanent cells which are developed 

 from them, and are more or less similar to one another; they are easily recognised 

 by their smooth thin walls, the quantity of protoplasm they contain, and the absence 

 of any coarsely granular deposits. Meristem may be again divided into the two fol- 

 lowing kinds : — 



{a) Primary Meristem comprises the whole cellular tissue of very young organs or 

 parts of organs, as the apices of roots and stems, the youngest leaves, and the embryo ; 

 from it are subsequently developed the later forms of tissue. We shall hereafter 

 discuss this form of tissue more in detail. 



{b) Secondary Meristem occurs in those organs or parts of organs which have 

 advanced beyond the condition of primary meristem, and therefore contain differentiated 

 forms of permanent tissue, among which the secondary meristem is usually found in 

 the form of thin layers, and furnishes, from its power of forming cells, the material 

 for the production of new permanent tissue in addition to that already in existence. 

 Various forms of secondary meristem will be hereafter described under the names of 

 Cambium, Thickening-ring, and Phellogen or Cork-cambium. 



It not unfrequently happens that the large cells of a permanent tissue which have 

 already attained a high degree of development, with moderately thick walls, a large 

 quantity of cell-sap, ^but a relatively small proportion of protoplasm, and which 

 contain chlorophyll-granules or other coarsely granular deposits, begin to grow 

 and to divide afresh ; as, for example, in the primary cortex of annual shoots and 

 stems when they increase rapidly in diameter. The cortical cells are compelled, by 

 the increase in size of the wood, to stretch in a tangential direction ; each breaks up 

 by repeated radial division into a number of segments (as shown in Fig. 56, p. 71); 

 and these, as soon as they are formed, behave like mature permanent cells. Among 

 plants of very simple structure such a division of more mature cells takes place in 

 Conjugatae, especially in Spirogyra (Fig. 13, p. 17). A tissue in this condition has 

 been termed Older Meristem, in contrast to Younger Meristem, in which all the 

 dividing cells are still in their youngest condition. 



(2) Permanent Tissue is the result of a further development of those cells of the 



