LEAVES AND LEAF-FORMING AXES. 



^37 



whether the expressions Leaf and Stem ought still to be used ; and when finally 

 the similarity preponderates, the whole shoot is no longer called a Leafy Stem, 

 but a Thallome. A branched thallome has the same relation to a leaf-bearing stem 

 as a slightly differentiated to a highly differentiated whole. 



The differentiation of the external forms of the members of the shoot into Stem 

 and Leaf is to a certain extent independent of the internal differentiation which brings 

 about the different forms of tissue and the cell-divisions, as is shown in the com- 

 parison of crosses and Characeae with Phanerogams. The internal segmentation may 

 be reduced to a minimum of cell-divisions, or may altogether disappear ; in the latter 

 case the single cell presents itself as a shoot, the lateral outgrowths of which behave 

 as leaves, and the axial part as stem, as, for example, in Caulerpa amongst Algae. 

 What has been already said as to the continuity of the tissue of the stem and leaf, 

 and its origin from the primary meristem, must here be understood in an extended 

 sense. In place of the primary meristem we have the pimctum vegetationis of a 

 single cell continuing its growth at the apex, and instead of the differentiation 

 of tissue the development of the older cell-wall and of its contents. Caulerpa 

 consists of a single cell- utricle, which grows as a creeping stem and puts out 

 lateral leaf-like protuberances and tubular hairs which perform the function of roots, 

 the whole enclosing a continuous cell-cavity without partition-walls \ 



(a) The leaves, like the shoots, grow at first at the apex, /. e. at the free end opposite 

 the place of their origin. This apical growth continues indefinitely in many thallomes 

 and leaf-forming axes until checked by some external cause ; this is especially the case 

 in the primary shoots of Fucacea", pleurocarpous Mosses, Characeae, the rhizome of 

 Equisetaceac, ?>rns, the main stems of Coniferae and many Angiosperms. If the primary 

 shoots themselves bear organs of reproduction, the apical growth generally ceases with 

 their development, as in many acrocarpous Mosses, the fruit-stalks of Equisetaceae, the 

 haulms of grasses which bear the inflorescence, and in all cases in Angiosperms where 

 a primary shoot ends in a flower. The lateral shoots are usually of limited growth ; 

 the growth frequently ceases without any external cause, and especially when they bear 

 reproductive organs, become transformed into spines, or are very different in their shape 

 from the primary shoot, as the horizontal lateral branchlets of many Coniferse, the leaf- 

 like shoots of Phyllocladus, Xylophylla, Ruscus, &c. 



In by far the greater number of cases the apical growth of leaves ceases early, the 

 apex itself becoming transformed into permanent tissue. In Ferns, however, the apical 

 growth of the leaves usually continues, and in many genera is even unlimited, the apex 

 of the leaf always remaining capable of development, and not becoming transformed into 

 permanent tissue, as in Nephrolepis ; in Gleichenia, Mertensia, Lygodium, and Guarea, 

 the growth of the apex of the leaf is, as in many shoots, periodically interrupted, and again 

 renewed in each period of vegetation. 



(b) Besides the apical growth, there always exists however, both in stems and in 

 leaves, an interstitial growth, the parts produced by the apical growth thus increasing in 

 size and becoming further developed. The development of the internodes of the stem 

 depends almost exclusively on this, as indeed is shown by the crowded position and the 

 shortness of the internodes in the bud ; the interstitial growth generally appears at first 

 very rapid, and the increase in size occasioned by it is often very considerable ; but it 

 usually soon ceases, and the tissues become differentiated into unchanging permanent 

 forms. Not unfrequently, however, a basal zone of internodes (as in Grasses, Equhetum 



* Cf. N.igcli, Zeitschiift fur wissenschaftliche Botauik und neiieie Algensysleme. 



