FUNDAMENTAL ORGANS. 131 



vessels and liber-fibres. The lower surface of the leaf, which corresponds to the 

 cortical system, is generally more hairy and presents more stomata than the upper, 

 which corresponds to the wood system. The parenchyma of the leaf, filled with 

 green chlorophyll, usually presents (fig. 697), in flat leaves, two well-marked 

 divisions ; the upper, belonging to the woody system, consists of one or more series 

 of oblong cells (p.s), arranged perpendicularly side by side beneath the epidermis 

 (e.s), leaving very small interspaces (m) ; the lower division, belonging to the 

 cortical system, consists of irregular cells (p.i), with interspaces (l) corresponding 

 with the stomata. The parenchyma of fleshy leaves (as Sedum) consists of cells 

 with feAV interspaces, which cells become poorer in chlorophyll towards the centre 

 of the leaf. Submerged leaves (fig. 698) have no epidermis, stomata, fibres, or 

 vessels ; their parenchyma is reduced to elongated cells, arranged in few series, 

 and is consequently very permeable by water. 



The leaf originates as a small cellular tumour, which afterwards dilates into a 

 blade, the cells on the median line of which 

 elongate and form fibres, then, as in the stem, 

 first trachese, and lastly other vessels. 



In his treatise ' On the Formation of 

 Leaves,' Trecul admits foiir principal types 

 according to which these organs are formed : 

 the hasifuqal, hasipetal, mixed, and parallel. 



T it- i. % 7 j-u 1 r • J 1 T ^ 698. Potamogeton. 



In the basVfuqaL, the leai is developed irom section perpendicular to the surface of a leat (mag.). 



,^ -1 . n 1T1 p, parenchyma witjjout.e'pidermia ; V, interstices. 



below upwards, i.e. the oldest parts are 



those at the base of the leaf, and the tip is the last part formed ; the stipules 

 appear before the leaflets and secondary nerves of the leaf. In the hasipetal 

 type, the rachis or axis of the leaf appears first, and on its sides the lobes and 

 leaflets spring from above downwards ; the tip is hence developed before the base. 

 The stipules are developed before the lowest leaflets, and sometimes even before the 

 upper. In this type, not only the leaflets, but their secondary nerves and teeth, 

 appear in succession downwards. In the mixed arrangement, both these types are 

 followed. In the parallel type the nerves are all formed in parallel lines, but the 

 sheath appears first. The elongation of the leaf takes place at the base of the blade, 

 or base of the petiole. The sheath, although the first formed, does not increase till 

 the leaf has developed to a certain extent. 



The nerves of leaves are arranged very difierently in monocotyledons and dicoty- 

 ledons. In the former (fig. 33), they are usually simple, or, if branched, the branches 

 do not inosculate. In dicotyledons, on the contrary (fig. 6), the nerves branch into 

 veins and venules, which inosculate with those of the neighbouring nerves, and form 

 a fibro-vascular network of which the interstices are filled with parenchyma. 

 Nevertheless, in some monocotyledons, the basal nerves are not all parallel and 

 simple ; but secondary nerves spring from one or more of the principal nerves, and 

 diverge in other directions ; but these secondary nerves are parallel, and the con- 

 vexity of the arc which they describe is turned towards the principal nerve (this 

 nervation is rare among dicotyledons) ; lastly, the nerves in monocotyledons may 



TL 2 



