62 TISSUES 



If a number of filaments, each the product of cell-division, arrange 

 themselves side by side, or become closely intertwined or interwoven, a 

 complex structure results which will resemble an ordinary tissue 

 formed by cell-division more or less closely, according as the filaments 

 are firmly united or only loosely combined : where complete con- 

 crescence takes place between the component filaments, the resulting 

 tissue is often hardly or not at all distinguishable from a tissue pro- 

 duced by division. An instance in point is provided by the genus 

 Cuthria among the Phaeosporeae. Here the thallus is a fiat structure 

 composed of several superimposed layers, each of which breaks up at 

 the margin into a number of narrow segments. If a single layer be 

 examined separately, it is .found that each marginal segment consists 

 of a cellular filament, which grows by means of an intercalary meris- 

 tem ; behind this meristematic zone lateral branches arise, and still 

 further back adjacent filaments unite so completely that the fully 

 developed tissue of the thallus shows no trace of its peculiar mode of 

 development. Various other Phaeosporeae, such as Desmareslia and 

 Sfilophora, form their tissues by the same [" trichothallic "J method, or 

 by means of a closely similar process. In the Ceramiaceae again the 

 main axis becomes corticated through the concrescence of closely 

 appressed lateral branches ; in Cham cortication of the mternodes is 

 effected in a somewhat similar fashion. 



Excellent illustrations of tissue-formation by concrescence and 

 coalescence of simple filaments are furnished by the " tissues " of the 

 Higher Fungi; these invariably consist of slender and often richly 

 branched cellular filaments or hyphae, which may be regarded as 

 histological elements of a higher order, as compared with the segments 

 of which they are composed. Hyphae may become associated to form 

 tissues in very various ways. They may be closely interwoven into, 

 felted membranous structures, or bound together to form strands of 

 approximately parallel filaments ; in other cases the arrangement that 

 eventuates strongly resembles the parenchymatous tissue of a Higher 

 Plant. 



It is customary to describe those tissues which arise entirely by 

 typical cell-division as " genuine " tissues, and thereby to contrast them 

 with the " false " tissues produced by coalescence and concrescence of 

 separate cells, cellular filaments or cell-masses ; in the same way the 

 various types of false tissue are distinguished by such names as 

 "pseudo-parenchyma," etc., their development being described as a 

 process of "apparent" tissue-formation. From the purely morpho- 

 logical point of view this classification is thoroughly justified; the 

 physiological anatomist, however, cannot recognise any such distinc- 

 tions, since the functional activity of a tissue is in no way determined 



