108 



BOTANY 



PART I 



The more rapid development of the leaf-tip is most striking in some tropical 

 plants, especially in climbers. In this case, according to M. RACIBORSKI, these 

 "fore-runner tips" perform the functions of the leaves before the remainder of the 

 leaf has attained the mature condition. 



Well-marked and long-continued apical growth is found in the leaves of some 

 Ferns. 



Welwitschia mirabilis behaves in a peculiar way unlike all other cormophytes. 

 Above the cotyledons only a single pair of foliage leaves is formed. The basal 

 zones of these grow in each annual period while the ends of the leaves are gradually 

 withering. 



2. Different Forms of Leaves. The leaves of the shoot have very 

 diverse functions and are correspondingly various in their form on the 



same stem, although in their 

 origin they are alike. 



The main axis of the 

 seedling bears first the COTY- 

 LEDONS or seed-leaves which 

 are situated on the hypocotyl 

 (Fig. 158) of the embryo 

 while it is yet in the seed. 

 In the Monocotyledons there 

 is only one such leaf, while 



FIG. 125. Development of the leaf in the Elm, Ulmus the Dicotyledons and SOme 



campestris. A, Showing the vegetative cone, v, with ^ , 



the rudiments of a young leaf, 6, still unsegmented, C*ymilOSpermS have two 



and of the next older leaf, exhibiting segmentation cotyledons and SOHie GymilO- 



into the laminar rudiment, o and leaf-base g B sperms haye more t h an two> 



Showing the older leaf, vie wed obliquely from behind. * . 



(x 58. After STRASBURGER.) Following On the Cotyledons 



in the case of subterranean 



stems, and often also in those above ground, come a number of SCALE 

 LEAVES (Fig. 123 nd), then in the case of aerial shoots the FOLIAGE 

 LEAVES (lb) y and still higher simply formed BRACTEAL LEAVES (Aft). 

 The foliage leaves may be first considered since the other forms have 

 arisen by transformation of these. 



A. The Foliage Leaves exhibit a great variety of form and 

 segmentation, and these characters are largely employed in descriptive 

 botany. They may be simple as in the needles of Coniferae ; in this 

 case the primordial leaf has only to increase in size and lengthen. As 

 a rule, however, the foliage leaf is segmented into the flattened, thin, 

 bright-green LEAF-BLADE (lamina, Fig. 124 sp), which is often 

 inaccurately spoken of as the leaf ; the stem-like LEAF-STALK (petiole, 

 Fig. 124 s) ; and frequently also into the STIPULES (nb) attached to 

 the LEAF-BASE close to the stem or into a LEAF-SHEATH (vagina, Fig. 

 133 v) more or less completely surrounding the stem above the node. 

 When the leaf -stalk is wanting the leaf is termed sessile; when 

 present it is petiolate. The segmentation is recognisable at an early 

 stage in the primordial leaves, which are differentiated shortly after 



