204 The Aiiatomv of Plants BOOK.H 



layers from which the principal system of tissues is derived. 

 Such layers arise in different ways in different cases, and 

 at various distances from the apex.' l 



We have discussed Hanstein's proposals at some length 

 on account of the fundamental importance claimed by the 

 scheme he promulgated. It was not a mere classification of 

 tissues but an attempt to give a complete and scientific 

 account of the underlying principles of development. In 

 this respect it had much in common with another hypo- 

 thesis promulgated later by Van Tieghem, to which con- 

 siderable attention must be devoted. Before, however, 

 these new views obtained currency, the not very satis- 

 factory nature of Hanstein's position led to the proposal 

 to classify the tissues on another basis, which, if not so 

 scientific, would enable adequate descriptions of their struc- 

 ture to be given. The most noteworthy of such descriptive 

 systems was that put forward by Sachs in the Lehrbuch. 

 It had, however, a distinctly scientific foundation, though 

 of a different character from that of Hanstein. Whereas 

 the latter was mainly ontogenetic, Sachs' classification 

 was chiefly physiological, being based on the function of 

 the different tissues. He distinguished epidermal, fibro- 

 vascular, and fundamental tissues, all being derived from 

 a uniform primary tissue or meristem at the apex ; the 

 epidermal included the protective layers which bounded 

 the internal mass, the second the conducting strands, while 

 the third formed the remainder of the structure, through 

 which the conducting tissues pass, and which supports 

 them and aids in maintaining their individuality or indepen- 

 dence. The epidermal tissues thus comprised more than 

 the epidermis of Hanstein, which was the adult condition 

 of the dermatogen, extending backwards from the growing- 

 point. They included also the secondary formations making 

 their appearance below the epidermis proper as growth and 

 1 Science Progress, 1894, p. 473. 



