118 



BOTANY 



PART I 



differentiated than these when mature, being usually scale-like and 

 sessile. They are developed by enlargement of the primordia, 

 mainly from the leaf -base, while the lamina remains more or less 

 undeveloped (Fig. 124 1-6, Fig. 137). Scale leaves, either colourless 

 or green, often occur on the aerial shoots before the foliage 



leaves (Fig. 1 2 3 nd). They 

 are also the only foliar 

 organs on rhizomes, appear- 

 ing as hardly visible and 

 usually short-lived scales, 

 while in accordance with 

 the development in dark- 

 ness foliage leaves are 

 wanting (Fig. 123 ws, Fig. 

 138). The bracteal leaves, 

 on the other hand, re- 

 semble in construction 

 the scale leaves on aerial 

 shoots, but are often vari- 

 ously coloured and tend to 

 succeed the foliage leaves 

 as the subtending leaves 

 and bracts of the flowers 

 or inflorescences. The in- 

 ternal structure of both 

 scale leaves and bracts is 



FIG. 136.-Seedling of Acacia pycnantha. The cotyledons s i mp l er than that of the 



have been thrown off. The foliage leaves 1-k are pinnate, ,. ,. r , mi , ,, 



the following leaves bipinnate. The petioles of leaves foliage leaves. 1 hey hardly 



5 and 6 are vertically expanded ; and in the following take part in the nutritive 



leaves, 7, 8, 9, modified as phyllodes, bearing nectaries, nmpp p <, V,,^ arft ncm a llv 

 n. (About i nat. size. After SCHENCK.) 'CCSSCS, C USUally 



protective structures for 



the young leaves or the buds. They are, however, connected with 

 the foliage leaves by intermediate forms (Figs. 124, 137). 



That scale leaves and bracts are to be regarded as arrested forms of foliage 

 leaves is shown not only by the developmental history but by the possibility 

 of deriving foliage leaves from their rudiments or primordia. Thus GOEBEL 

 succeeded in causing leaf primordia that would have formed scale leaves to become 

 foliage leaves by removing the apex and stripping the leaves from the shoots. 

 Subterranean stems, when forced to develop in the light, form foliage leaves from 

 the primordia which in the earth would have become scale leaves. In their 

 internal structure, however, the scale leaves and bracts are not merely arrested 

 foliage leaves but frequently exhibit special differentiations connected with their 

 particular functions ( 65 ). 



3. Duration of Life of Leaves. In many plants the leaves have 

 a shorter life than the stems on which they are borne. The leaves in 

 such plants are shed from the stems (LEAF-FALL) or, in the case of 



