CIRCULATION AND FORMATION OF WOOD IN PLANTS. 591 



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 fenestrated 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 ex 

 pect 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 vas 

 cular 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 parietes 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 resist 

 ance. 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 it 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 cor 

 responding 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 

 individually 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 frame 

 work of veins, can neither contract easily as a mass, nor allow its sep 

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

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

 and having their flexible sides surrounded by air, may severally have 

 their contents considerably increased and decreased without offering 



