IBel 



LEAVES AND LEAF-BEARING AXES. 157 



(7) Every Leaf assumes a form different to that of the Stem which produces it, and 

 to that of its lateral Shoots. This is usually so conspicuous that no further descrip- 

 tion is needed. Nevertheless one point must be mentioned which often causes 

 difficulty to the student. It not unfrequently occurs that lateral shoots of certain 

 plants present a great similarity in form and physiological properties to the foliage- 

 leaves of other plants, as the flat lateral shoots (phylloclades) which bear the flowers 

 in Ruscus, Xylophylla, Muhlenheckia platyclada, &c. ; but the course of development 



hows that these apparent leaves must, from their position, be lateral shoots, them- 

 Ives producing leaves ; and the leaves of these plants are usually of quite a dif- 

 ferent 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-bearing lateral 

 shoots arise in very difl"erent ways in diff'erent plants; but very commonly they 

 have this in common with leaves, — that they originate also as lateral and exogenous 

 outgrowths in the primary meristem of the growing point ; that they are formed, 

 like the leaves, in acropetal succession; and that the diff'erentiation of their tissue 

 proceeds continuously with that of the primary shoot. They are distinguished, 

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

 leading 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 is there- 

 fore a repetition of the primary shoot, although in other physiological characters 

 it may diff"er greatly from it. 



(8) The morphological conceptions of Stem 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 ap- 



' plicable 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 together all the structures which we call leaves without reference to 

 the stems to which they belong, we shall be unable to find a single characteristic 

 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 Phyllome 

 and Caulome cannot be obtained by comparing together the positive properties 



I of leaves or the positive properties of stems, or by la}dng stress on the points 

 which they have in common and on those wherein they differ; but these ideas 

 ^ 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 on a stem as 

 lateral structures. 



2 [Warming (Ramification des Phanerogames, p. xvii) remarks that while it is impossible to find 



constant characters for separating phyllomes from caulomes, they spring from the peripheral tissue 



at slightly different depths. Phyllomes originate in the superficial layers of the periblem, from the 



first to the third; feebly developed foliar organs, such as bracts, even in the first layer alone. 



1^ Caulomes scarcely ever originate in the first layer, but usually in the third or fourth. Warming 



^K attributes this to the necessity for the largest structure to have the deepest origin.] 



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