SYSTEMS OF TISS^UES. 



?»3 



differentiatioii of the forms of tissue out of the homogeneous embryonal tissue 

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

 forms and. systems of tissues: in their developed coiidition. 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 ftinctions 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 poirits when discussing their physiological, significance. 



If we imagine any plant whatever consisting of cell-tissue, and living in water, 



FIG. III.— Fructification of Bolehis JliiTjidus in Ion. 

 gitudinal section, and slightly magnified, st stipes ; hti 

 piieus; hy hymenium; w velum; h tlie cavity beneath^ 

 tlie hymenium; /"continuation of the hymenial layer 

 on the stipes; ht the removable yellow skin^of the 

 piieus. 



Fig. 112. — Transverse section of the stem ofBryiiTn 

 reiseum (X 90). w root hairs, produced by the out- 

 growths of single cells' of the external layer. 



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 subserves the absorption of nutritive matters, the formation 

 of epiderniis will obviously be less conspicuous than in shqot-axes and leaves 

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



[3] . I ' 



