596 APPENDIX C. 



vessels at first by the spiral or allied vessels originally developed, 

 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- 

 perimentallyprovedthat the same intermittentcompressionsproduce 

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

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

 have sufficed of themselves to develope the supporting and distribut- 

 ing structures of plants, is not alleged. What share the natural 

 selection of variationsdistinguishedas spontaneous, has had in estab- 

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

 acting alone natural selection would have sufficed to evolve 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 in- 

 dividuals on which such other cause had acted most favourably. 

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

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



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

 natural selection as having here played the part of an accelerator. 



EXPLANATION OF PLATE. 



Fig. 1. Absorbent organ from the leaf of Euphorbia 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 Ficus 

 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 Panax 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 much more massive absorbent from the 

 same leaf, the surrounding tissues being omitted. 



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

 from the leaf of Clusia fava. 



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

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

 funnel-shaped interior is drawn as it presents itself 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, showing its annuli of reticulated cells when cut through. 

 The cellular tissue which fills the interior is supposed to be removed. 



