8o 



MORPHOLOGY OF TISSUES. 



which is not yet dilTerentiated may be termed Primary Tissue, or, since its cells 

 are always capable of division, Primary Meristem ^ 



In the present section a separate paragraph is devoted to each tissue-system, 

 and a description will first be given of the various forms of cells and tissues which 



are constituents of all systems. Those 

 forms which are peculiar to one or 

 another system will be discussed in 

 their proper place. 



(a) In reference to external form, 

 the following cell-combinations may 

 be enumerated : — 



(i) The term Ttssue may be ap- 

 plied par excellence to aggregations of 

 similar cells which, without any well- 

 defined external form, consist, in what- 

 ever direction the section be made, of 

 numbers of cells. Illustrations of such 

 tissues occur in the greater number 

 of the larger Fungi ; the fundamental 

 tissue of thick stems of Ferns ' and 

 Monocotyledons is a parenchymatous 

 tissue penetrated by other forms of 

 tissue in the shape of strings. In 

 Dicotyledons the pith especially comes 

 under the same denomination ; it may 

 also be found in the purest form in the 

 flesh of succulent fruits. In stone- 

 fruits, such as the peach, plum, cocoa- 

 nut, &c., the stone consists of a 

 sclerenchymatous tissue. It may also 

 occur that forms of tissue which 

 differ morphologically may be aggre- 

 gated into a mass of tissue uniform 

 in its physiological characters, as in 

 the secondary wood of trees and the succulent tissue of tubers, such as the potato, 

 dahlia, &c. 



(2) A row or Filament of Cells is composed of similar cells placed singly side by 

 side or in rows ; but they are usually connected genetically. Isolated filaments of 

 this character occur in the hyphge of P\mgi and in many Algae, where they have 

 arisen by transverse partition of an apical cell or by intercalary transverse division. 

 In the higher plants we have abundant illustrations: — in the epidermaF hairs, vessels^ 

 laticiferous vessels, sieve-tubes, tannin-receptacles (as in the phloem of Phaseolus)^ &c. 

 wi.ich are found in the interior of tissues. 



(3) A simple Layer of Cells results where similar cells are so united in one plane 

 that the entire layer is only a single cell in thickness. Among Cryptogams it is not 



Fig. 67. — Transverse section of the stem of SelagUiella inceqicali- 

 folia. The outer layers of cortical have thick dark-coloured cell-walls; 

 the thinner-walled fundamental tissue envelopes three fibro-vascular 

 bundles, separated from it by large intercellular spaces I {X 800). 



* It may not be superfluous to remark that the pith and cortex are neither forms nor 

 systems of tissue, but are altogether indefinite and undefinable; we speak, for example, of 

 cortex in Thallophytes in quite a different sense to what we do in Vascular Plants ; the cortex of 

 Monocotyledons is something different from that of Conifers and Dicotyledons; in the latter the 

 cortex has quite a different signification in young and in older parts of stems. The same is the 

 case with the pith, 



