158 
MORPHOLOGY OF MEMBERS. 
are obtained by observing leaves exclusively in their relation to the stera 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, Sargasstwi). 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 differentiadon which brings 
about the different forms of tissue and the cell-divisions, as is shown in the 
comparison of Muscineae 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 as leaves 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 hke a 
creeping stem and puts out lateral leaf-like protuberances and tubular hairs which 
even 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, 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 Fucaceae, pleurocarpoiis Mosses, Characese, the rhizomes of 
Eqiiisetaceae, 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 Nägeli, Zeitschrift für wissenschaftliche Botanik, and Neuere Algensysteme. 
