THE THREE SYSTEMS OF TISSUES. II5 



Thus, with a little consideration, and bearing in mind the most general vital 

 conditions of the plant, we find that there is a triad of tissue-systems which 

 compose the bodies of highly organised plants ; or, put better, three systems of 

 tissues arise in these plants by differentiation. These are the epidermal tissue, the 

 fibro-vascular strands or vascular bundles, and the remaining tissue, which I have 

 already distinguished from the other two as the fundamental tissue. It has 

 recently been attempted, it is true, to resolve these three systems of tissues 

 into a greater number of different forms of tissues ; in this, however, either quite 

 subordinate physiological points of view, or even purely formal morphological con- 

 siderations have been started from. I find, after all that has been said in this 

 direction in the literature of the last six or eight years, no ground whatever for 

 giving up the classification into the three systems of tissues named, which is, moreover, 

 almost universally accepted ; and so much the less, since all other histological con- 

 siderations can be harmonised with these three systems in a perfectly unconstrained 

 manner. This is possible on the one hand even with very highly organised vascular 

 plants, where, at least in the young states of the organs, epidermal tissue, vascular 

 bundles, and fundamental tissue are at once prominent as the three essential systems ; 

 on the other hand, I do not know how this triple division is to be evaded in 

 the Mosses, especially in the more highly organised true Mosses, as well as in the 

 more highly developed Algoe. The last named subdivisions of plants present, rather, 

 very fine examples of rudimentary forms of the three systems of tissues. If we 

 take into consideration, finally, the forms reduced or degraded from the highly 

 developed types, such as phanerogamic water-plants and parasites, it again results 

 that all other differentiations of tissues fall completely into the background as 

 secondary matters in contrast to those here brought forward. 



As in the consideration of the external segmentation of plants, I shall here 

 also introduce in the first place the typical highly developed forms, in their most 

 important relations of configuration, and then come back to some rudimentary 

 and derived or reduced forms. 



We find the typical forms of the three systems of tissues, however, developed 

 chiefly in the axes and leaves of aerial shoots, and the following statements are 

 in the first place intended to refer to these. 



The Epidermis is the superficial tissue of the younger shoot-axes and leaves, not 

 yet altered by the requirements of an advanced age ; since in true roots a specially 

 organised epidermis can but rarely be spoken of, and even in submerged 

 water-plants the outer layer of tissue is often scarcely to be distinguished from 

 those lying deeper. In its typical form, the epidermis is thus found on the 

 leaves and shoots exposed to air and light, and is but little modified even on 

 those underground. Physiologically understood, the epidermis is, as already 

 mentioned, the layer of tissue sharply marking off the organ concerned from 

 the exterior, and its essential properties chiefly amount to this, — to hinder the 

 evaporation of the fluids contained in the cell-tissue, and to prevent injurious 

 influences entering from without; while, on the other hand, through more or 

 less numerous stomata, a means is afforded of discharging the aqueous vapour 

 ■formed in the interior, according to need, and at the same time of facilitating 

 the entrance and exit of carbon dioxide and oxygen gas. We may regard 



