565 



reasels — at first by the spiral or aJlied vessels originally dercloped, 

 and then by the better-placed ducts formed later. By experiment 

 it is demonstrated that the intermittent compressions caused by os- 

 cillations urge the sap along the vessels and ducts. And it is also ex- 

 pei'imentaUy proved that the same intermittent compressions produce 

 exudation of sap from vessels and ducts into the surrounding tissue, 

 Tliat the processes here described, acting through all past time, 

 have sufficed of themselves to develope the supporting and distribnt- 

 ii!g structures of plants, is not alleged. What share the natural 

 iclection of variations distinguished as spontaneous, has had in, estab- 

 lishing them, is a question which remains to be discussed. Whether 

 acting alone natural selection would have sufficed to ei'olve these 

 vascular and resisting tissues, I do not profess to say. That it has 

 been a co-operating cause, I take to be self-evident : it must all along 

 have furthered the action of any other cause, by preserving the m- 

 dividuals on which such other cause had acted most favourably. 

 Seeing, however, the conclusive proof which we have that another 

 cause has been m action — certainly on individuals, and, in all proba- 

 bihty, by inheritance on races — we may most philosophically ascribe 

 the genesis of these internal structures to this cause, and regard 

 natm'al selection as having here played the part of an accelerator 



EXPLANATION OF PLATE. 

 Fig. 1. Absorbent oi'gan from the leaf of Euphorhia neriifolia. 

 The cluster of fibrous cells forming one of the terminations of the 

 vascular system is here imbedded in a solid parenchyma. 



Fig. 2. A structure of analogous kind from the leaf of Ficw 

 elastica. Here the expanded terminations of the vessels are im- 

 bedded in the network parenchyma, the cells of which unite to form 

 envelopes for them. 



Fig. 3. Shows on a larger scale one of these absorbents from 

 the leaf of Fanax Lessonii. In this figure is clearly seen the way in 

 which the cells of the network parenchyma unite into a closely- fitting 

 case for the spiral cells. 



Fig. 4. Represents a m uch more massive absoi'bent from the same 

 leaf, the surrounding tissues being omitted. 



Fig. 5. Similarly represents, without its sheath, an absorbent from 

 the luaf of Clusiajiaca. 



Fig. 6. End view of an absorbent organ from the root of a 

 Turnip. It is taken from the outermost layer of vessels. Its fuimel- 

 shaped interior is drawn as it presents itseU when looked at from the 

 outside of this layer, its narrow end being directed towards the 

 centre of the Turnip. 



Fig. 7 A longitudinal section through the axis of another such 

 organ, showmg its annuli of reticulated cells when cut tlirougii. Tlw 

 cellular tissue which fills the interior is supposed to be j-emoved. 



