THE EPIDERMAL TISSUE. yg 



and to designate by it cells united in both a parenchymatous and prosenchymatous man- 

 ner, when they are not only thickened, but also hard. We should then have Scleren- 

 chyma in cork, in fundamental tissue (as the dark strings in the stem of Pteris aquilina, 

 or the stone of stone-fruit), and in wood ; the indurated cells in the flesh of pears 

 would also come under this name. In one word, by this term would be designated not 

 a tissue-system, but only a physiological property of particular cells of a tissue-system \ 

 If the cells of a tissue are all or mostly capable of division, it is a Generating Tissue 

 (Meristem of Nageli) ; if they are not it is a Permanent Tissue. The primary tissue of 

 the youngest parts of plants is always a IMeristem, and may be distinguished as Primary 

 Meristem. In the older parts of plants portions of the tissue also remain merismatic 

 or become so subsequently ; they may be designated Secondary Meristem. At one time 

 this tissue was designated Cambium; but it is convenient to retain this word in its 

 original signification for that merismatic layer in the tissue of older parts of plants by 

 means of Avhich the increase in thickness of Dicotyledons and Conifers is accomplished. 

 The arrangement may produce a simple row or line of cells, in contrast to a cell-surface 

 where the cells form a lamella consisting of a single layer. If the cells are united in 

 all directions, we have a Tissue with dimensions in three directions. When the latter 

 is greatly elongated in one direction, and its growth proceeds especially at one or both 

 ends, and it lies inside another tissue, it is a Fascicular Tissue ; the cells of such a 

 tissue are usually elongated in the direction of its length, mostly prosenchymatous, and 

 we then have Prosenchyma bundles. The most important form of these are the Fibro- 

 'vascular Bundles which are dispersed through the fundamental tissue of the higher 

 Cryptogams and Phanerogams, whose cells are mostly elongated and partly prosenchy- 

 matous, and are thus formed into vessels, /. e. long rows of lignified cells, the septa of 

 which have been broken through. 



(b) The youngest parts of stems, extremities of roots, leaves, and other organs, 

 consist almost entirely of Primary Meristem ; as they become more perfectly developed 

 a separation may be recognised into layers of tissue and bundles, which represent the 

 commencement of the tissue-systems ; within each system its different forms of tissue 

 become gradually differentiated. When different tissue-systems in a mature condition 

 are in contact, the history of development alone can often determine whether certain 

 layers belong to one or the other system ; especially also because similar cell-forms 

 occur in different systems. Thus, for instance, parenchyma and prosenchyma, scleren- 

 chyma and secondary meristem may arise both in the fundamental tissue and in the 

 fibro-vascular bundles ; in the layers lying beneath the epidermis it often cannot be 

 determined whether they belong to the epidermal tissue or to the fundamental tissue 

 which bounds it. In the same manner also different forms of glands, vesicular vessels, 

 laticiferous ducts, resin and gum passages, occur in all three systems or in the funda- 

 mental tissue and the fibro-vascular bundles. The forms of cells and tissues here 

 named cannot be considered equivalent to these three tissue-systems ; they occur rather 

 as constituents of different systems. Nevertheless, on account of their physiological 

 peculiarities, I shall consider them together in a special paragraph, while the other more 

 important forms of tissue will be treated of under the three systems. 



Sect. 15. The Epidermal Tissued — A differentiation into epidermal tissue 

 and inner fundamental tissue can evidently only arise in plants and parts of plants 



' Cf. Otto Buch, Ueber Sclerenchymzellen. Breslau 1870. 



'^ By the introduction of the idea of the Epidermal Tissue into general use, as I here employ it, 

 a real want will, I think, be remedied in histology. In any case a series of histological facts, which 

 have hitherto been treated of in a detached manner, will thus be brought under a common and 

 higher point of view. 



