CHAP. 11. 



LEAVES. 



115 



lobes are tliemselves lobed, we may say tripejinatisected, tri- 

 pennatiparted, &c. Penally, in cases where leaves are extremely 

 divided, and the parenchyma of the ultimate ramifications of 

 the veins does not unite and form lobes, we say, in general 

 terms, that the leaf is midtifid^ laciniated, decomposed, or slashed ; 

 terms which express the appearance of a leaf, without any 

 very precise signification." 



With regard to compound leaves, their leaflets always 

 have the primary veins running at an angle more or less 

 acute towards the margin. " This is perfectly intelligible if 

 we reflect that their lateral veins represent not the primary, 

 but the secondary and tertiary veins of simple leaves, which 

 latter are always pennated. 



" The leaflets of pennated leaves are usually placed opposite 

 each other in pairs along a common petiole. These pairs of 

 leaves are called in Latin ju(/a : thus a leaf with one pair is 

 unyugum; with two pairs, hijugum, &c. 



" Usually one of the leaflets terminates the petiole ; the 

 leaf is then unequally pinnated {imparipinnatum) ; but sometimes 

 there is no odd leaflet, and the petiole ends abruptly, or in a 

 point or tendril ; (this is equally pinnated, pari-pinnatum), 



" Sometimes the leaflets themselves are subdivided into 

 other leaflets {folium bipinnatum, trijyinnatum). In this case, 

 the lateral petioles which bear the leaflets are called partial ; 

 and the small supports of the leaflets themselves, stalklets 

 {pefiolules)." 



Such are De Candolle's ideas of the typical formation of 

 leaves. They offer a convenient mode of studying the modi- 

 fications in structure of these organs, and are, to all ap- 

 pearance, founded upon a 55 

 correct idea of the subject. 



The PETIOLE, or leafstalk 

 {fg. 56. a — b), is the part 

 which connects the blade 

 with the stem, of which it 

 was considered by Linnaeus 

 as a part. It consists of one 

 or more bundles of fibro- 

 vascular tissue surrounded 



I 2 



