122 GHOWTH. 



which Dicotyledonous plants exhibit remarkable excep- 

 tions. 



179. Woody fibre is to be found in the pith of Mira- 

 bilis, and spiral vessels in that of Nepenthes ; and in the 

 latter stem there are no concentric woody layers, nor 

 medullary rays, and spiral vessels occur close to the 

 modified bark. In the Peppers which are not arbores- 

 cent, the wood exists as scattered or separate bundles 

 of tissue for the whole period of life of the plant, whilst 

 in those arborescent, according to Meyen, during the 

 second year of growth a complete ring of wood is 

 formed at the circumference of the stem, but within 

 which the wood exists as separate bundles as before : 

 and we are much indebted to Dr. Lindley for making 

 known several remarkable cases in which there is the 

 most curious departure from the normal appearance of 

 the Exogenous stem ; but in regard to these, as well as 

 to all cases of abnormal development, the remark of that 

 author should be carefully remembered, that " the part 

 next the centre is but little affected," and " that the 

 presence of a central pith, and a greater degree of hard- 

 ness in the wood next the centre than in the circum- 

 ference," are the signs from which we may conclude an 

 Exogenous growth. 



180. The increase of size in an Exogenous stem 

 takes place by the deposition of a new zone of wood on 

 the outside of the one last formed, and therefore it is 

 said to grow from without, or to be Exogenous. 



181. Now what is the origin of the woody layer by 

 means of which the increase of growth takes place ? 

 We must confess that at the present moment this ques- 

 tion is unsettled, for valid objections appear to exist 

 against what must be considered as even the most 

 probable views of the subject. We shall notice these 

 only, and the objections they are open to. If we 

 examine the stem of a plant in spring, we shall find be- 

 tween the bark and the wood a layer of viscid matter 

 called by Du Hamel, cambium, and reproductive layer 



