EXOGKNS. 



EXOOEKa 



Bit 



atque Saurureis,' p. 40.) This, if correctly described, only shows that 

 in certain Exogens a portion of the central tissue is placed at first in a 

 confuted manner, and that the wood doe* not assume a definite circu- 

 lar disposition till afterwards ; that it does assume it eventually is 

 admitted. We find in Piper nigntm and P. LonchVit that from the 

 beginning the woody bundles are placed circularly, but they are sepa- 

 rated by a good deal of cellular tissue, and do not assume in the first 

 one the wedge-like or triangular form which is most common in 

 Kxogens, and which they themselves at last take on. In Boerhaaria 

 rrponda, a specimen of which U now before us, we find the wood 

 regularly disposed in two zones, and instead of spiral vessels a very sin- 

 gular structure in the pith, which is filled with fistular passages of 

 lax soft spheroidal cellular tissue, surrounded by smaller, harder, and 

 more cubical cellular tissue which passes off into the medullary pro- 

 cesses. It is in irach plants as Piper incannm that the organisation of 

 Exogens most nearly approaches that of Endogens ; but in the first 

 place the whole race of Pipers forms a sort of transition from Exogens 

 to Araceous Endogens ; and secondly, it is probable that when they 

 are most endogenous in appearance they are not really so in regard' 

 to the final development of their woody tissue. 



Let it however be admitted that in certain cases Exogens are in 

 the centre of their stem organised less regularly than usual ; this will 

 offer no argument in favour of their analogy with Kmlogens. In all 

 Hiich cases it will be found that they eventually assume their typical 

 conformation. We are acquainted with some striking proofs of this. 

 Among twining plants of tropical countries we occasionally find 

 instances like the following : 



Beneath a most irregularly-compressed and lobed bark there lies a 

 mass of wood, apparently so confused and irregular in its arrangement 

 in the centre, that nothing symmetrical can be made out by the most 

 acute observer ; but it will be seen that towards the circumference it 

 distinctly assumes the radiated appearance of an Exogen. In other 

 cases, where the structure is sufficiently regular, this circumstance is 

 still more distinctly illustrated. 



It is however more commonly at the centre that wo look for typical 

 structure, and at the circumference that we find irregularity ; as if 

 Exogens usually commenced their growth according to the plan to 



which nature has subjected them, and only deviated from it under 

 the influence of unknown causes coming into operation and controlling 

 their development after they have advanced to a certain stage in their 

 growth. Thus, in the singular instances shown in cuts D, E, F, and 

 O, the principal part of the stem U so confused and irregular as to 

 look more like an Endogen than an Exogen, and a fragment might 

 easily be mistaken for the former ; nevertheless in a young and tole- 

 rably regular shoot (D) the radiated appcnrancc is sufficiently well 

 marked ; and in two others, irregular and ilintortcd an they are (E and 

 U), the central pith is visible, although far out of the centre ; and in 

 the fourth (P) the centre has not only pith, but a radiated structure 

 that is quite regular. 



B 



By far the most singular case of this sort is in an unknown twining 

 plant in the possession of Dr. Lindlvy, from the Halayau Archipelago, 

 of which the cuts H and I are representations. In old stems of this 

 plant a section exhibits a most irregular combination of wood, looking 

 like palm wood, broken up into lobed cords lying amongst still more 

 irregular cellular tissue, and inclosed in a common bark; so that we 

 doubt whether it would be possible to tell to which class it really 

 belongs, if it were not for its young shoots and the pith of the old 

 ones. The latter may be seen lying quite out of the centre towards 

 one side (near the bottom of our figure, a little to the right) ; and in 

 the former (H) the pith is found with wood radiating around it, 

 although still with sufficient irregularity. 



II 



The cases already given are evidences of exogenous wood being 

 sometimes extremely different from the condition in which we nee it 

 in Europe, and attest the necessity of forming our ideas of its nature 

 from a more extended examination than tlmt which is commonly 

 given to it Several curious cases have been also published by Dr. 

 Litnllcy in his ' Introduction to Botany ' (ed. 2, p. 77, lie.), and others 

 have been noticed by Schlcidcu and other wi ; 



Irregularity in the structure of exogenous wood is usually owing 

 either to a confused disposition of the tissue at some particular period 

 of the growth, or to some derangement of the medullary processes, 

 or to the absence of concentric circles, or to the formation of a deep 

 tone of cellular tissue altornati'ly with iwh /imi- "f wood, or, finally, 

 to the production of wood within the bark instead of beneath it. The 

 first cause has been already sufficiently illustrated. 



