4 THE PLANT CELL. 



(The leaves of Piiius and Halea belong to the centric type.) In 

 some leaf-structures, such as those of mosses, a much simpler 

 arrangement obtains, the leaf being, perhaps, only two or three 

 cells thick, and the vascular system quite rudimentarj-. 



The subdivision of the tissues of stem or root-structures in 

 plants into epidermis, cortex, and central cylinder, occurs typically 

 in the Dicotyledons and Coniferee, and these tissues are set apart 

 early in the young stem or root; in the Monocotyledons and the 

 Higher Pteridopliyta (ferns) is found the arrangement noted 

 in (c) (see supra) — viz., an external epidermis, and internally a 

 ground-tissue in which lie several separate vascular bundles, no 

 well-marked central C3'linder existing, although in the young 

 shoot a central cylinder may be detected. 



In plants below the Pteridophyta the main grouping of tissues 

 into external and internal may often hold good, but the diflFerentia- 

 tion is not so marked as it is in the higher types, and, finally,, 

 when the Fungi and Algfe are considered, the vascular tissues 

 cease to exist ^;er sc, and the plant becomes a structure known as 

 a ThaUus (Thallophyta) the component tissues of which conform 

 to one or at most a few simple types, and are not always to 

 be differentiated into internal and external groups. 



With regard to the general nomenclature of tissues, those 

 in which the component cells have equal, or nearly equal, dimen- 

 sions whichever way they are measured, are termed paren- 

 chyma ; whilst those where the cells have an elongated shape, 

 one dimension being possibly ten or twenty times the other, 

 are known as prosenchyma. Amongst the latter are scleren- 

 chyma, bast fibres, cambial elements, and elements of the 

 xylem and phloem, all of which will be examined in detail 

 (Chaps, iv. and x.). 



The functions of the cells in the various tissues will be to a 

 certain extent studied together with their structural details, but 

 broadly speaking it may be here stated that the following 

 tissues — viz., epidermis, cortex, and the mesophyll of leaves — 

 function in assimilation, transpiration, and elaboration of food 

 materials, the epidermis being also often protective in nature; 

 the cork, collenchyma and sclerenchyma are mainly protective, 

 and confer elasticity and rigidity upon an organ in which they 

 are present ; whilst the wood and bast confer rigidity, and are 

 essentially concerned in the conduction of sap (the phloem 



