l^H MORPHOLOGV OF MEMBERS. 



are obtained by observing leaves exclusively in their relation to the stem which 

 produces them, and stems in relation to the leaves produced from them. In other 

 words, the expressions Stem and Leaf denote only certain relationships of the parts 

 of a whole — the Shoot ; the greater the differentiation, the more clearly are Stem 

 and Leaf distinguished. The measure of the difference is usually arbitrary; but 

 if we confine ourselves to those plants to which the term leaf is applied in ordinary 

 language, the distinction of leaves from stem depends on the relationships named 

 in paragraphs (1-7); and in this sense certain lateral outgrowths in some Algae 

 may be termed Leaves, and the axial structures which produce them Stems {e.g. 

 Characeae, Sargassum). But when the difference between the outgrowths and the 

 axial structures which produce them is less, one or more of the relationships named 

 in paragraphs (1-7) disappear, and it becomes doubtful 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 external differentiation 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 

 comparison of Muscinese and Characeae with Phanerogams. The internal seg- 

 mentation may be reduced to a minimum of cell-divisions, or may altogether dis- 

 appear; in the latter case the single cell represents a shoot, the lateral outgrowths 

 of which behave asjeaves and the axial part as stem, as, for example, in Caulerpa 

 amongst Algae. What has already been said as to the continuity of the tissue 

 of stem and leaf and their common origin from the primary meristem, must 

 here be understood in an extended sense. In place of the primary meristem we 

 have the growing point of a single cell continuing its growth, and instead of 

 the differentiation of tissue the development of the older part of the cell-wall 

 and of its contents. Caulerpa consists of a single cell, which grows like a 

 creeping stem and puts out lateral leaf-like protuberances and tubular hairs which 

 even perform the function of roots, the w^hole enclosing a continuous cell-cavity 

 without partition- walls \ 



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

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

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

 in the primary shoots of Fucacese, pleurocarpous Mosses, Characeae, the rhizomes of 

 Eqiiisetacege, Ferns, and the primary stems of Coniferae and of 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 fertile stems 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, more 

 especially when they bear reproductive organs, or become transformed into spines, or 

 are very different in their shape from the primary shoot, as the horizontal lateral 

 branchlets of many Coniferae, the leaf-like shoots (phylloclades) of Phyllocladus, Xylo- 

 phylla, Ruscus, &c. 



^ See Nageli, Zeitschrift fiir wissenschaftliche Botanik, and Neuere Algensysteme. 



