HEEBERT SPBNCEr'S BIOLOGY. 381 



to be found. Wlien leaf structure comes to be more fully worked 

 out, we doubt not that other cases will come to light, showing the 

 close resemblance between leaf and branch. The ConifercB promise 

 good results in this way, e.g. the leaves of Fhyllocladus, and those 

 of Araucaria, which latter are said to produce a layer of woody 

 bundles for two or three years in succession. At present, acting 

 under the supposition of the distinctness of axis and appendage, we 

 have to assume two different ways in which the fruit may be formed ; 

 from carpellary leaves alone, or from a dilatation or hollowing out 

 of the upper part of the flower-stalk. So, too, in the case of the 

 placenta, we are compelled to consider the placentation as some- 

 times axial, at other times foliar. 



Again, there is the nucleus of the ovule and its coats, the exact 

 morphological nature of which is far from being understood. Take, 

 for instance, the ovule of Welwitschia, and its outer covering. Is 

 the latter of foliar or of axial origin ? At present, the balance of 

 evidence is in favour of the latter view, as also in the case of Taxa- 

 cecB* The ovuliferous scales of Finusy etc., lie under the same 

 uncertainty. The Podostemacece too, or many of them, are all but 

 destitute of roots, and their stems fulfil the functions and have the 

 appearance of leaves. The difficulties that exist in the deciphering 

 of the morphological nature of these and many similar cases that 

 might be cited, would, we feel sure, be considerably lessened if it 

 were at once acknowledged that no absolute distinction between stem 

 and leaf existed. In the present state of our knowledge, the most 

 important distinction that can be pointed out, or rather the one 

 least liable to exception is this, that buds may be formed on the 

 surfaces, or edges, or in the axils of leaves, while none are ever pro- 

 duced from its apex. On the other hand, it is the rule, not the 

 exception, for the stem to terminate in a bud. The extremity of the 

 axis is formed by a cone of growing cellular tissue, from which leave3 

 may originate, while at the extremity of the leaf is a layer of dead, 

 functionless cells. f But to revert to our author, Mr. Spencer, as 

 we think very judiciously, looks for the beginning of the perfect 

 exogenous or endogenous stems among acrogenous plants. It has, 



* N.H.R. April, 1863. 

 f For an account of the structure and composition of the stem, the reader is 

 referred to papers of M, Germain de St. Pierre, Bull, Soc. Bot. France, 1855, p. 159, 

 et 1 860, p. 5 ; to those of Hanstein, Ann. Sc. Nat. 4th Ser. Vol. viii. p. 5, and Linnsea, 

 xxxi. p. 65, and to that of E. Meyer, Linnjea, xvi. p. 402. 



