78 



MORPHOLOGY OF TISSUES. 



of different layers of tissue, systems of tissue-forms, which may be designated simply 

 2LsSys/ems 0/ Tissue. We thus usually find an Epidermal System, a Fascicular System, 



and the system of the Fundamental 

 Tissue between them (Fig. 67). But 

 whenever a differentiation of tissues 

 of this kind arises in a plant, it 

 only takes place subsequently ; ori- 

 ginally the whole mass consists of a 

 growing portion of the plant (stem, 

 leaf, root), always of a uniform 



'Z/^^^^^M^i^O^MM^i^Z^^'yK' ' I \~Hiff tissue, out of which by diverse de- 



velopment of its layers these tissue- 

 systems have their origin ; this tissue 

 of the youngest parts of plants which 

 is not yet differentiated may be 

 termed, in opposition to the others, 

 Primary Tissued 



(a) Within each tissue -system the 

 cells may be formed and arranged in 

 very different ways; their contents and 

 their cell-wall may be differently de- 

 veloped ; in each system they may be 

 capable or incapable of division^. If 

 the cells are pointed at the ends, and 

 much longer than they are broad, and 

 at the same time their ends penetrate 

 between one another so that no inter- 

 cellular spaces occur, then the tissue is 

 termed Prosemhyma. If, on the other 

 hand, the cells are arranged in rows, bounding one another with broad surfaces, thin- 

 walled, not much longer than broad, and forming intercellular spaces, they form a 

 Parenchyma. The two forms of tissue pass over into one another in many ways, 

 and in the use of the term Parenchyma a painful uncertainty prevails in vegetable 

 anatomy. There are forms of tissue which cannot be included under either of these two 

 terms, if they are made to possess any definite signification ; as, e.g.^ the tissue of Fungi and 

 Lichens, and even of Fucaceae. In parenchyma as well as in prosenchyma the cells may 

 be thick or thin walled, lignified or not, the contents may be succulent or may consist 

 of air. It would be convenient to generalise the term Sclerenchyma used by Mettenius, 



Fig. 67.— Transverse section of the stem oi Selasinella itiaqiialifoha. 

 The cell-tissue, consisting of several layers of cells, has dark thick cell- 

 walls ; the thinner-walled fundamental tissue envelopes three fibro-vas- 

 cular bundles, separated from it by large intercellular spaces (/) (x8oo). 



^ It may not be superfluous to remark here, in the meantime, that pith and cortex are neither 

 forms nor systems of tissue, but altogether indefinite and undefinable ideas; we speak, e.g., of cortex 

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

 cotyledons 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. 



^ An enumeration of the nomenclature of tissues would here be of no service. In elucidating 

 the facts, as I do partly in the following paragraphs partly in Book II, I shall employ the technical 

 terms as they are required by a consideration of the different objects and relationships. I keep, 

 with a few deviations, to the terms and distinctions proposed by Nageli (Beitrage zur wiss. Botanik, 

 Heft I, 1858). 



