PARENCHYMA. XI5 



SECTION III. 



PARENCHYMA. 



Sect. 25. The term Parenchyma is here applied to all internal cellular tissue, i. e. 

 that which is found within the epidermis or cork-layer. Though as a fact it is in the 

 main identical with Sachs' ground-tissue (p. 6), in conception it is not so. 



It has already been stated (p. 5) that in this limitation and classification of tissues 

 the distinction is drawn between those elements which retain their cell-nature, and such 

 as have lost that character. Attention was also drawn to the difficulty in the way of 

 a generally uniform distinction, partly from the incompleteness of our present know- 

 ledge, partly from the undoubted occurrence of real intermediate forms between cellu- 

 lar tissue and many distinct tissues, especially Sclerenchyma. These difficulties did 

 not come prominently forward in Sections r and 2. Here, in the internal tissueSj 

 they appear frequently, and it is everywhere to be repeatedly pointed out that in the 

 distinctions about to be drawn, definite types must be indicated, which recur univer- 

 sally, but are never sharply distinguished from one another. As regards the distinction 

 of cells from other tissue-elements resulting from the metamorphosis of cells, it may 

 here be again called to mind that the former are distinguished from the latter by the 

 permanent protoplasmic body, in which the nucleus also (always?) remains, or 

 appears temporarily. With these parts, which are directly observable anatomically, 

 the cells retain the faculty of active growth and of division : it is true that this 

 faculty is often enough not manifested, but, in the processes of secondary thickening 

 (comp. Chap. XV), and especially in the phenomena of formation of cork (Sect. 24) 

 brought about by wounding, it may be so generally observed, that it may serve as 

 a very useful character. The chlorophyll-containing parenchyma of a foliage-leaf, 

 for instance, after complete unfolding shows as a rule no further division : the 

 smallest wound immediately induces it. In very thick-walled, sclerotic cells, the direct 

 anatomical determination of protoplasm and a nucleus is difficult, and as a fact is 

 often impossible. Nor is that of the power of division more practicable. In its stead 

 another phenomenon is to be taken into account, namely the periodic appearance 

 and disappearance of starch-grains in many elements, which judging from the nature 

 of their walls may be doubtful. Putting out of account the sieve-tubes (Chap. V), 

 and certain laticiferous tubes (Chap. VI), in which, at all events, peculiair conditions, 

 which need not here be touched upon, are the rule, the formation of starch in all 

 well-known cases is directly connected with an active protoplasmic body. In 

 doubtful cases therefore it is to be regarded as a character which indicates the 

 presence of such a body, so long as it is not proved that it can also occur in spaces 

 without protoplasm, and surrounded by cell-membranes. Abundant starchy con- 

 tents, and especially periodical changes in their amount, must therefore for the 

 present be regarded as a criterion of the cell-quahty. In Chap. XIV we shall again 

 return to this subject. 



Respecting the structure of the cells of the parenchyma, nothing general need 

 be brought forward at present, which would not be included in the doctrine of 

 the structure of the mature vegetable cell, and this we assume to be already known. 



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