iiOS ON ThE PROGRESSIVE DEVELOPMENT OF 



which a longitudinal section will present a three-sided prism, 

 and which will extend from the base to the summit of the flower- 

 stalk. But if within each secondary cell a certain number of 

 tertiary vesicles be developed, so as to form cells of considerable 

 size and nearly empty, we shall then have the structure of the 

 central part of certain trunks of the monocotyledons, and also that 

 of the pith, as it occurs in the stems of some of the dicotyledons. 

 Now if, instead of vesicles thus developed, parallel and con- 

 centric cells are formed, we shall have a commencement of 

 concentric layers, and other vesicles being developed in their 

 interstitial spaces in a longitudinal direction, so as to form tubes 

 or vessels, a transverse section will exhibit the perfect develop- 

 ment of the concentric vascular tissue. If the cells, instead of 

 loose hexagonal cells, give rise to other vesicles pressed one 

 against another, and developed on the coats of more internal 

 cells which are incased in and line completely the cavity of 

 those in which they are formed, then we shall have the most 

 complicated organization of the stem of a dicotyledonous plant, 

 with its incased and concentric layers, and its medullary rays, 

 or divergent layers, which are nothing else than the interstices 

 of the secondary cells, or rather the junctions of their sides, in 

 which vesicles have been developed in a horizontal direction, 

 forming tubes or vessels running from the centre of the stem 

 towards the circumference.* 



Branches are nothing more than the divisions of the stem, 

 and may be conceived to be formed by the progressive develop- 

 ment of cells and tubes in a manner altogether similar, — a 

 generating vesicle gradually developing other vesicles upon its 

 internal membrane, and the growth taking place as in the former 

 instance. 



Leaves consist of a cuticle or transparent film of cellular tex- 

 ture, enclosing a parenchyma or pulp, also of cellular tissue, 

 and usually of a green colour, and fibres of which the nerves 

 and veins of the leaf are chiefly composed. These fibres com- 

 municate with each other, and are ramifications from the central 

 nerve or midrib. In the monocotyledonous plants, the nerves are 

 for the most part parallel, or nearly so, extending from the leaf- 

 stalk to the apex of the leaf in a longitudinal direction, and 

 rarely branched, but communicating with each other by short 

 transverse ribs, which gives the appearance of a tesselated or 

 rectangular network to the vascular structure. In the acotyledons, 

 the leaves or leafy expansions are destitute of ribs or veins, and 

 consist merely of a pulpy cellular tissue, enveloped by a cuti- 

 cular expansion. The development of these parts from the 

 simple vesicle or cell may be thus supposed to arise. Upon the 

 internal surface of the envelope of a primary cell, two secondary 

 vesicles may follow the growth of the generating or primary cell, 

 and stretch out longitudinally; they will thus form two lateral lobes 



* Raspail. Elements of Organic Chemistry, by Henderson, p. — (?) 



