THE TISSUE SYSTEMS 13 



the situation in this respect for the monocotyledons, which may 

 on strong grounds be considered as the highest of the seed plants. 

 Externally is the piliferous layer, from which the root hairs are 

 derived, and which may be considered in a general way as the equiva- 

 lent of the epidermis of the stem and the leaf. The central region 

 is occupied by the fibrovascular system, sharply limited by the 

 endodermis, composed of cells ordinarily thick-walled. Between 

 the endodermal limiting membrane and the piliferous layer lies 

 the cortex of the root, and this corresponds to the fundamental 

 category of tissues in the case of the stem and leaf. 



It will be clear from the account given above that there are three 

 tissue systems in plants which are very distinct in lower forms and 

 in the less changeable and more conservative parts. Of these the 

 epidermal tissues are always clearly limited both toward the out- 

 side and also in relation to the tissue system which lies inside. 

 The boundaries dividing the fibrovascular from the fundamental 

 tissues are often less plainly indicated, and in the case of the higher 

 groups of plants, particularly those in which the secondary growth 

 is strongly developed, may disappear altogether; in such cases the 

 limits of the tissues can only be inferred from comparative and 

 developmental anatomy. 



