SECT. I 



MORPHOLOGY 31 



and theM completes its segmentation and development by intercalary 

 growth, which is usually localised near the base. It is true that 

 some leaves, such as those of Ferns, have apical growth and continue 

 to grow for a long time (in some cases, e.g. Lygodium, the growth is 

 unlimited), and bear their pinnae in acropetal order. On the other 

 hand, the leaf-like cladodes, although they are in reality metamor- 

 phosed shoots, exhibit a limited apical growth like that of ordinary 

 leaves. 



We have already seen that in certain cases the apical cone may give rise to a 

 terminal leaf. The apex of the leaf in many ferns [e.g. Adiantuvi Edijcworthii) may, 

 on the other hand, be transformed into a bnd from which a shoot will develop. 



The un segmented protuberance of the still rudimentary leaf, termed 

 by EiCHLER (12) the primordial leaf (Fig. Zl A h), first projects from 

 the vegetative cone of the 

 shoot {A v). This is usually 

 followed by a separation of 

 the primordial 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 ^ jj 



which immediately adjoins 



, . . ■, Fio. 3-2.— Apex of an Elm shoot, Ulmiis cnmpestrts. A 



the vegetative cone, eitlier showing the vegetative cone i', with the rudiments 



takes no further part in the of a young leaf &, still unsegmented, and of the next 



succeeding differentiation of o^^er leaf, exhibiting segmentation into the laminar 



" . J , . rudiment, o, and leaf-base, g ; B, showing the older 



the leaf, or it develops into leaf, viewed obliquely from behind, (x 58.) 



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), this is later interposed by intercalary growth between 

 the leaf-blade and the leaf-base. 



The leaf-tip often develops more rapidly than the rest of the leaf, and Goebel 

 regards this as an arrangement for the protection of the younger parts of the 

 bud ('■*). This is seen most strikingly in some tropical plants, especially in climbers. 

 In this case, according to M. Raciborski {^^), the " fore-runner tips " serve for 

 assimilation before the remainder of the leaf has attained the mature condition. 



In leaves the laminae of which do not remain simple, but undergo segmenta- 

 tion as they develop, the lateral segments are as a rule formed in the basipetal 

 direction, i.e. from the tip towards the base (Fig. 33) ; the opposite direction of 

 development is, however, sometimes found. The segments of the palmate and 

 pinnate leaves of Palms originate by a process of splitting of the leaf-blade which 

 is, to begin with, entire. The direction of the splits is determined by the folds 

 which the lamina assumes when young ("''). 



Different forms of leaves are most clearly seen in the Phanero- 

 gams, in which the various leaf structures found at different regions 



