DISTINCTION BETWEEN MEMBERS AND ORGANS. 



^53 





(c) General ideas, like those considered here and in the sequel, depend always on 

 abstractions ; they therefore necessarily want the practical clearness of the particular 

 ideas from which they have been abstracted. How far the abstraction may be carried is 

 more or less arbitrary ; and the only correction for this lies in the scientific usefulness 

 of the idea. Those ideas are the most useful which, from the greater precision of 

 the definition, and from their greater clearness, include the greatest possible number 

 of particular cases ; for in this manner is that complete general comprehension of the 

 phenomena most easily obtained which must precede a closer examination of them. The 

 definitions in the following paragraphs are given from this point of view. 



Sect. 21. Leaves and Leaf-bearing Axes^ — The members of the plant 

 which are called Leaves (Phyllomes) in Characeae, Muscineae, Vascular Cryptogams, 

 and Phanerogams, are related to 

 the axis or stem from which they 

 are derived in the manner de- 

 scribed in the following para- 

 graphs. 



(i) T/ie Leaves ahvays origi- 

 te below the groiving apex of the 

 ieni as lateral outgrowths, either 

 singly, or several at the same 

 height, i.e. at an equal distance 

 from the apex ; in the latter case 

 they form a whorl, the single 

 leaves of which may differ in age, 

 as in Chara and Salvinia, and in 

 the whorls of many flowers. 



(2) So long as the apex of 

 the shoot continues growing in a 

 straight line, and the portion of 

 the shoot which produces leaves 

 lengthens, the leaves arise in 

 acropetal order ; i. e. the nearer 

 the leaves are to the apex, the 



younger they are. In this case leaves are never produced further from the apex 

 than those already in existence. It is only when, as not unfrequently happens 

 with the flowers of Phanerogams, the growth in length of the shoot ceases or 

 becomes weaker at the apex, while, at the same time, active growth continues in 

 a transverse zone or place beneath the apex, that new leaves can arise between 

 those already in existence ^. 



(3) The Leaves always originate frofn the Primary Merisiem of the Growing 



Fig. 116.— Longitudinal section through the apical region of a stem of 

 Foiitinali'! antipyretica, an aquatic Moss (after Leitgeb) ; v the apical cell 

 of the slioot, producing three rows of segments which are at first oblique 

 and afterwards placed transversely (distinguished by a stronger outline). 

 Each segment is first of all divided by the septum a into an inner and 

 an outer cell ; the former produces a part of the inner tissue of the stem, 

 the latter the cortex of the stem and a leaf. Leaf-forming shoots arise 

 beneath certain leaves, a triangular apical cell ^ being formed from an outer 

 cell of the segment, which then, like v, produces three rows of segments ; 

 and each segment here also forms a leaf. (For a more exact description see 

 Book II, Mosses.) 



^ Nageli u. Schwendener, Das Mikroskop. Leipzig 1869, p. 599 et seq. — Hofmeister, AUge- 

 meine Morphologic der Gewebe. - Leipzig 1868, Sect. 2. — Pringsheim, Jahrb. fiir wissen. Bot. 

 vol. III. p. 484. — Ditto on Utricularia, in Monatsber. der Berliner Akad., Feb. 1869, — Hanstein, Bot. 

 Abhandlungcn, Bonn 1870, Heft I. — Leitgeb, Botan, Zeitg. 18 71, no. 3.— Warming, Recherches sur 

 la ramification des Phanerogames. Copenhagen, 1872, p. vi. 



^ Since phenomena of this kind are confined to the flowers and inflorescence of Phanerogams, 

 their consideration may for the time be postponed. 



