S y STEMS OF TISSUES. II3 



differentiation of the forms of tissue out of the homogeneous embryonal tissue 

 of growing-points and embryos; it suffices for our purpose to understand the 

 forms and systems of tissues in their developed condition. Even in this, however, 

 I confine myself to the most necessary points. It only concerns , us at first to 

 describe the tissues and systems of tissues generally distributed in the vegetable 

 kingdom : the description of all those histological matters which subserve special 

 physiological functions may be deferred until I have to speak of the latter themselves. 

 We might otherwise at this period easily deteriorate into a dry formalism and a 

 wearisome systematic arrangement, which will best be avoided by treating of the 

 various points when discussing their physiological significance. 



If we imagine an}- plant whatever consisting of cell-tissue, and li\ing in water, 



FIG. III.— Fructification o( HfMiis ßavüiiis in lon- 

 gitudinal section, and slif;htly magnified, st stipes ; >tii 

 pileus ; /ty hymenium; f velum ; h tlie cavity beneath 

 hymenium; y^ continuation of the liymenial layi 



Fig. 112.— Transverse section of the stem o( Br_ 

 eitm {X 90). 7u root hairs, produced by the 

 nvths of single cells of the external layer. 



the 



s; ht the ren.ovable yelk 



pi! 



earth, or air, the first thing demanded of its organisation is an efficient shutting 

 off" of the tissue-masses from the surrounding world : an external layer of cells, or 

 according to circumstances several such layers, obtains greater solidity and other 

 peculiarities, by means of which this shutting off is effected. Thus there arises 

 in the first instance the differentiation of an epidermal tissue as distinguished from 

 the inner mass of tissue; and it need hardly be added that the contrast between 

 the two is the more marked the more important is the actual shutting off from 

 without. In the roots of land-plants and in the shoots of submerged water-plants, 

 where the whole surface sub.serves the absorption of nutritive matters, the formation 

 of epidermis will obviously be less conspicuous than in shoot-axes and leaves 

 which live in tlie air, and have to be protected above all against the loss of 

 [3] • I 



