CHAPTER TIL 



MINUTE STRUCTURE AND DEVELOPMENT OF THE ROOT, 

 STEM, AND LEAF OF PH^NOGAMOUS PLANTS. 



GENERAL CONSIDERATIONS. 



305. THE tissue elements, described in the preceding chapter, 

 are arranged in various waj'S to form and connect the organs of 

 the plant, If elements of the same kind are united, the}' consti- 

 tute a tissue, to which is given the name of those elements ; thus 

 parenchyma cells form parenchyma tissue or simply parenchyma ; 

 cork-cells form cork, etc. A tissue can therefore be defined as 

 a fabric of united cells which have had a common origin and 



O 



have obeyed a common law of growth. 



Tissues are united to form systems; systems, to form organs. 



306. In nearly all plants with which the present treatise deals 

 there is some kind of framework consisting mainly of the more 

 elongated cells and ducts. This framework runs throughout 

 the entire organism. It is surrounded by parenchyma, in which 

 other tissue elements may also occur ; the epidermis in some of 

 its modifications covers the whole. 



307. The three chief systems found in plants are, therefore, 

 the fascicular, the cellular, and the epidermal ; and these corre- 

 spond in a general way to three classes of functions. In the 

 cellular system are found the active cells Ijy which assimilation, 

 the proper work of the plant, is effected ; the fascicular system is 

 largely conductive, and serves also important mechanical ends ; 

 the epidermal system brings the assimilative apparatus of the 

 plant into safe relations with the surroundings. 



No discussion of the cellular and epidermal s^-stems, intro- 

 ductory to a special consideration of them as they occur in the 

 different organs, is needed ; but some general statements relative 

 to the fascicular system will obviate repetitions later. 



308. The fascicular system, in its most complete development, 

 comprises the following tissue elements, which occur in very 

 different proportions in different cases, - -prosenchyma in the 

 widest sense, including wood-cells of all kinds, ducts, fibres, and 

 cribrose-cells ; together with some commingled parenchyma. With 



