46 



PART I. THE MORPHOLOGY OF PLANTS. 



[12. 



The growth in length of the primordial leaf is at first apical in 

 all cases ; it may be persistently apical {e.g. Ferns, generally) ; 

 or apical growth may be early arrested, further elongation being 

 effected by basal growth {e.g. Iris, Onion, Myriophyllum, Potentilla 

 anserina) ; or, more rarely, basal and apical (e.g. Acliillea Mille* 

 folium, and other Composite) growth may occur simultaneously. 



A characteristic feature of leaves is that their growth in length 

 is limited ; but this is not without exception ; in fact, there are 

 all intermediate forms between those which have limited and those 

 which have unlimited growth. Thus, in most Phanerogams the 

 leaves have limited growth; the cells of the leaf are all actually 

 formed at the time of its unfolding, and all that takes place sub- 

 sequently is that the cells grow to their definitive size. In a few 

 of these plants, however, (e.g. Guarea and 

 other Meliaceae) the pinnate leaves have an 

 apical growing-point by which new cells are 

 formed, and the growth in length of the leaf 

 and the development of lateral branches is 

 carried on after the leaf has unfolded. Long- 

 continued apical growth appears to be the 

 general rule in Ferns : in Pteris aquilina and 

 in Aspidium Filix Mas the leaf grows for 

 three years; and in Gleichenia, Lygodium, 

 many Hymenophyllaceae, and Nephrolepis, the 

 leaf grows for many years after its appear- 

 ance above the soil. The most striking ex- 

 ample of long-continued basal growth is that 

 of the two leaves of Welwitschia which persist 

 and grow basally as long as the plant lives, 

 and consequently attain a great lengih. 

 The leaves are inserted upon the nodes (p. 21) of the stem, the 

 plane of insertion being usually transverse to the longitudinal 

 axis of the parent stem ; but in some Bryophyta (Blasia, Schisto- 

 stega) the plane of insertion is parallel to the longitudinal axis of 

 the stem. 



The Hypopodium or Leaf-Base. The leaf-base commonly de- 

 velopes into a cushion of tissue, termed the pulvinus, which forms 

 the articulation by which the leaf is attached to the stem ; in the 

 Gooseberry the pulvinus developes into a spine. In many cases 

 the leaf-base is sheathing, and embraces a part or the whole of the 

 circumference of the node : in the former case the leaf is said to 



FIG. 27.-Leaf of Ra- 

 nunculus Ficaria: t> leaf- 

 base (hypopodium) ; p 

 petiole (mesopodium) 

 I lamina (epipodium). 



