DEVELOPMENT OF LEAVES AND BRANCHES. 



471. 



so-called upper leaf, buds forth only on one side. If the leaf is subsequently seg- 

 mented into petiole and lamina, which of course does not always happen, the petiole 

 is to be regarded as a structure subsequently interposed between the leaf-base and the 

 lamina, a subject to which we shall return in the next lecture. 



In the INIosses and Vascular plants the leaf-rudiments generally arise so closely 

 above and by the side of one another, that none of the free surface of the growing- 

 point at all is exposed beneath the youngest leaf. Only through the further course 

 of development in the second period of growth is it then decided whether the leaves 

 which arise closely one over the other are separated from one another by the 

 secondary intercalation of interfoliar parts on an elongating shoot-axis, or whether 

 this does not occur ; in the latter case none of the free surface of the shoot-axis at all 

 appears, even in the fully grown condition, because the 

 leaves, situated close above and below one another, 

 occupy the entire surface of the shoot-axis, as for 

 example in the common Fern Aspidium and in the 

 short twigs of many trees, and the so-called radical 

 rosettes of many biennial Dicotyledons. Cases where 

 the leaves at the growing-point are from the first 

 so far distant from one another that naked inter- 

 foliar parts of the shoot-axis exist between them, are 

 probably extremely rare ; as an example may be 

 mentioned the Bracken Fern {Plej-is Aquilina), where 

 the growing-point of the horizontal stem annually pro- 

 duces only one leaf, which requires two years more 

 for its completion. In the leaf-forming Algae also the 

 leaves usually arise quite close above and by the side 

 of one another on the shoot-axis, even though very long 

 interfoliar parts separate them subsequently, as is very 

 clearly the case in the Characese (cf. Fig. 95, p. 96). 

 The genus Caulerpa, on the other hand, presents a 

 good example of the other case (Fig. 308), where the 

 growing-point v of the shoot-axis always first elon- 

 gates to a considerable extent before a new leaf 

 buds forth from it again. We meet with similar 

 conditions of affairs in those Algae also where the 

 lateral outgrowths of the growing-point of a shoot do not develope into flat leaves 

 in the ordinary sense, and where therefore the shoot constitutes a mere branch 

 system, as in Fig. 309, where it may be noticed that the youngest branches appear at 

 some distance apart from the very first. 



Shoots, however, produce not only leaves and other organs, but new growing- 

 points of shoots generally arise again from their growing-points ; that is, they 

 branch. 



Only in very few plants does no branching take place — that is, no formation of 

 growing-points of secondary shoots. The best known examples of these are found 

 in some small acrocarpous Mosses, and above all in a few Vascular Cryptogams. 

 The longest known instance of this is the genus Isoetes, the stem of which grows 



'f^^ 



HG. 309.— Branching of a 

 leafless shoot of the Alga Gcli- 

 riiiim (one of the Floridere). 



