J ^6 EXTERNAL CONFORMATION OF PLANTS. 



that these apparent leaves must, from their position, be lateral shoots, themselves 

 producing leaves; and the leaves of these plants are usually of quite a different 

 form from these leaf-like branches. The phrase 'leaf-like' has in these cases 

 usually no distinct morphological, but only a popular meaning ; and what will be 

 said under paragraph (8) may be applied here. The branches or leaf-forming lateral 

 shoots arise in a very different manner in different plants ; but very commonly they 

 have this in common with leaves, that their origin is equally in the primary meristem 

 of the piincium vegetationis as lateral and exogenous outgrowths ; that they are 

 formed, like the leaves, in acropetal succession ; and that the differentiation of their 

 tissue proceeds continuously with that of the mother-shoot. They are distinguished, 

 however, from the leaves of the same plant by the place of their origin, by their 

 much slower growth, — at least at first (later they may overtake the leaves), — and 

 by their relations in point of symmetry, of which we shall speak hereafter. The 

 leadino- fact, however, is that the lateral shoot repeats in itself, by the formation 

 of leaves, all the relations hitherto named between leaf and stem, and appears 

 therefore as a repetition of the mother-shoot, although in other physiological rela- 

 tionships it is very different. 



(8) The morphological concepHojis of Skin and Leaf are correlative; one cannot 

 be conceived without the other ; Stem (Caulome) is merely that which bears Leaves ; 

 Leaf (Phyllome) is only that which is produced on an axial structure in the manner 

 described in paragraphs 1-7 ^ All the distinguishing characters which are applicable 

 to the definition of Caulome and Phyllome express only mutual relationships of 

 one to the other ; nothing is implied as to the positive properties of either. If 

 we compare with one another all the things which we call leaves, without refer- 

 ence to the stems to which they belong, we are unable to find a single charac- 

 teristic which is common to them all and which is wanting in all stems. But that 

 which is common to all leaves is their relation to the stem. Hence the ideas Phyl- 

 lome and Caulome cannot be obtained by comparing with each other the positive 

 properties of leaves and the positive properties of stems, or by laying stress on 

 the points which they have in common and on those wherein they differ. But 

 these ideas are obtained by observing exclusively leaves 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 rela- 

 tionships 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 many Algae may be termed Leaves, and the axial structures which 

 produce them Stems {e. g. Sargassum). But when the difference between the 

 outgrowths and the axial structures which produce them is less, one or several of 

 the relationships mentioned in paragraphs 1-7 disappear ; and it becomes doubtful 



^ There are, for instance, thallomes strikingly similar to certain leaf-forms, as those of Lami- 

 naria, Delesseria, &c. ; they are, however, not leaves, since they are not formed pn a stem as 

 lateral structures. 



