MORPHOLOGY 29 



short time only, and then completes its segmentation and develop- 

 ment by intercalary growth. It is true that some leaves, as those of 

 Ferns, not only continue growing for a long time, but also retain 

 a continuous apical growth and complete their whole segmentation in 

 acropetal succession. On the other hand, the leaf-like cladodes, 

 although they are in reality metamorphosed shoots, exhibit a limited 

 apical growth like that of ordinary leaves. 



Leaving out of consideration the Ferns and a few related plants, the 

 following observations in regard to the development of the leaf hold 

 good for the majority of Cormophytes. The unsegmented protuber- 

 ance of the still rudimentary leaf, termed by Eichler the primordial 

 leaf (Fig. 30, A, b), first 

 projects from the vegetative 

 cone of the shoot (A, v). 

 This is usually followed by 

 a separation of the primor- 

 dial leaf into the leaf- 

 base (g in A and B) and 

 the rudimentary lamina or 

 upper leaf (o in A and B). 

 The leaf-base, or the part of 

 the rudimentary leaf which FlG - 30 -— A P ex of an Elm shoot > ulm " campestns. a, 



i • , 1 -i • • ,i Showing the vegetative cone v, with the rudiments 



immediately adjoins tne of a y0UI)g leafi 6> still mlse g mented] and olthe next 



vegetative COne, either takes older leaf, exhibiting segmentation into the laminar 



110 further part in the SUC- raiment, °- /«<• leaf-base, g; B, showing the older 

 . r . leaf, viewed from the side, (x 5S.) 



eeedmg differentiation of the 



leaf, or it develops into a leaf-sheath (vagina) or into stipules. 

 The upper leaf, on the other hand, gives rise to the leaf-blade or 

 LAMINA. If the fully-developed leaf possesses a leaf-stalk (petiole), 

 it becomes afterwards interposed by intercalary growth between the 

 upper leaf and the leaf-base. 



The metamorphosis of the leaf is exhibited in its greatest diversity 

 by the leaves of Phanerogams, in which the various homologous leaf 

 structures have been distinguished as scale leaves, foliage leaves, 



BRACTEAL LEAVES, and FLORAL LEAVES. 



Foliage Leaves, generally referred to simply as leaves, are the 

 leaf structures on which devolves the task of providing nourishment 

 for their parent plants. As the exercise of this function is dependent 

 upon the presence of a green pigment, foliage leaves have, accordingly, 

 a green colour. In certain cases, where their form is extremely simple, 

 as in the needles of Conifers, the primordial leaf simply increases in 

 length without any further differentiation into parts. In other un- 

 divided leaves, however, whether lanceolate, elliptical, ovate, or other- 

 wise shaped, the flat leaf lamina is distinct from the leaf-base, while a 

 leaf-stalk may also be interpolated between them. If no leaf-stalk is de- 

 veloped the leaf is said to be sessile, otherwise it is described as stalked. 



