500 



office, in the common Turnip-root. As shown by the end view 

 and longitudinal section in figs. 6 and 7, this organ consists of 

 rings of fenestratcd cells, arranged with varying degrees of regu- 

 larity into a funnel, ordinarily having its apex directed towards the 

 central mass of the Turnip, with which it has, in some cases at least, 

 a traceable connexion by a canal. Presenting as it does an external 

 porous surface terminating one of the branches of the vascular sys- 

 tem, each of these organs is well fitted for taking up with rapidity 

 the nutriment laid by in the Turnip-root, and used by the plant 

 when it sends up its flower-stalk. Nor does even this exhaust the 

 analogies. The cotyledons of the young bean, experimented upon 

 as before described, furnished other examples of such structures, 

 exactly in the places where, if they are absorbents, we might 

 expect to find them. Amid the branchings and inosculations of the 

 vascular layer running through the mass of nutriment deposited in 

 each cotyledon, there are conspicuous free terminations that are club- 

 shaped, and prove to be composed, like those in leaves, of irregularly 

 formed and clustered fibrous cells; and some of them, diverging 

 from the plane of the vascular layer, dip down into the mass of 

 starch and albumen which the young plant has to utilize, and which 

 these structures can have no other function but to take up. 



Besides being so well fitted for absorption, and besides being 

 similar to organs which we cannot doubt are absorbents, these 

 vascular terminations in leaves afford us yet another evidence of their 

 functions. They are seated in a tissue so arranged as specially to 

 facilitate the abstraction of liquid. The centripetal movement of the 

 sap must be set up by a force that is comparatively feeble, since, the 

 par'etes of the ducts being porous, air will enter if the tension on the 

 contained columns becomes considerable. Hence it is needful that 

 the exit of sap from the leaves should meet with very little resistance. 

 Now were it not for an adjustment presently to be described, it would 

 meet with great resistance, notwithstanding the peculiar fitness of 

 these organs to take 't in. Liquid cannot be drawn out of any 

 closed cavity without producing a collapse of the cavity's sides ; 

 and if .its sides are not readily collapsible, there must be a corre- 

 sponding resistance to the abstraction of liquid from it. Clearly the 

 like must happen if the liquid is to be drawn out of a tissue which 

 cannot either diminish in bulk bodily or allow its components indi- 

 vidually to diminish in bulk. In an ordinary leaf, the upper layer of 

 parenchyma, formed as it is of closely-packed cells that are without 

 interspaces, and are everywhere held fast within their framework of 

 vei'ns, can neither contract easily as a mass, nor allow its separate 

 cells to do so. Quite otherwise is it with the network-parenchyma 

 below. The long cells of this, united merely by their ends and 

 hiving their flexible sides surrounded by air, may severally have 

 their contents considerably increased and decreased without offering 



