INTRODUCTION. 3 



On passing upwards through the Fungi to the Liverworts and 

 Mosses, and so on to the Higher Plants, it is noticed first, that 

 the external configuration of a plant becomes, as a rule, more 

 complex, there being a subdivision of the whole into various 

 organs ; and secondly, that this subdivision coincides with a 

 correspondingly complex internal organisation ; it is found, in 

 fact, that where in the lower plants all the vital functions take 

 place in the one or perhaps a very few cells, in the higher plants 

 a division of labour obtains whereby separate functions are 

 relegated to as many separate tissues. 



In the higher plants in which well-marked organs, such as 

 stem, leaf, and root, are found, it will be seen that the cells 

 composing these organs may be grouped into tissues, which have 

 the following nomenclature and general arrangement from with- 

 out inwards : — 



Ste'rn or Root. 



External Tissues : (a) Epidermis. 



(J) Cortex;* at times continuous with (f). 



Internal Tissues : (c) A general fundamental tissue in which 

 lie the vascular bundles, or, 



{d) A well-marked central cylinder (bounded by a tissue 

 known as endodermis), which comprises the vascular system, 

 and the pith or medulla. 



Besides these main tissues, there occur in various positions in 

 the cortex, fundamental tissue, or central cylinder, certain other 

 tissues, which, as a rule, have a supporting or protective function ; 

 such are cork, collenchyma, and sclerenchyma, the occurrence 

 and features of which will be examined in due course (Chap. iii.). 



In leaf -structures, the tissue arrangement may be of two 

 kinds — viz., (a) the bifacial, or (i) the centric. In the former is 

 found externally the epidermis on both upper and under surfaces 

 of the leaf, and internally a tissue known as mesophyll, in 

 which lie vascular elements and at times other tissues of a sub- 

 sidiary nature. In the latter or centric type there is present a 

 general arrangement not unlike that found in the stem of the same 

 plant — viz., externally, epidermis and mesophyll; internally, 

 a central cylinder in which are to be seen the vascular bundles. 



*The cortex is only external with regard to the central cylinder. It is, 

 however, convenient to deal with it among the external tissues. 



