540 PHYSIOLOGY. 



or less extensive and complicated framework of fibro-vascular 

 bundles. 



In the leaf of tlie Turnip, for example (fig. 585), we find an upper (e) 

 and lower (V) epidermis, with an intermediate mass of parenchyma (or 

 merenchyma, m, m'), rather close in the upper part, whence the name 

 palisade tissue, and spongiform in the lower part. The epidermis is 

 studded with stomata (a), which open into intercellular spaces commu- 

 nicating freely throughout the spongy tissue, and, further, through the 

 petiole, with the intercellular passages in the stem. The fibro-vascular 

 system (ribs and veins) runs through the lower lax parenchyma (fig. 

 585, /.>), and consists of bundles of spiral vessels and liber, the former 

 continuous with the medullary sheath and youngest part of the vascular 

 axis of the stem, the latter continuous with the liber-bundle outside the 

 cambium. The primary ribs in most Dicotyledons contain much liber, 

 and thus become very thick, so as to project from the lower face of the 

 leaf. They have a structure almost precisely like that of a small branch. 



Great differences result from the different degrees of development of 

 the spongy portion, as may be seen by comparing the leaf of the Lilac 

 with that of the Aloe or Mesemlryanthcmitm &c. The degree of consoli- 

 dation of the epidermis bv the formation of thickening layers is the 

 principal source of difference in the degree of solidity of leaves. 



Submerged leaves of aquatic plants have no stomata nor any extensive 

 intercellular system ; the epidermis is also little developed, and there is 

 commonly a total absence of fibro-vascular tissue : hence the delicate and 

 perishable character of these organs. 



The leaves and other appendicular organs are especially the seat of the 

 glandular and analogous epidermal structures. 



Structure of Petioles. The petioles usually consist of a mass of paren- 

 chyma, surrounded by epidermis and traversed by fibro-vascular bundles 

 arranged in a more or less semicircular manner, the spiral vessels being 

 uppermost, corresponding to the medullary sheaths. When cylindrical 

 the bundles form a complete circle, and the structure is then undis- 

 tinguishable from that of a young branch, on which account the ordinary 

 leaf has been regarded as a branch the upper portion of whose vascular 

 tissues are suppressed or depauperated (C. deCandolle). 



Fall of the Leaf. The fall of the leaf, as of the fruit, and in some 

 cases of the branches (as in Taxodium, &c.), is effected by the gradual for- 

 mation of a layer of thin-walled cells across the petiole, at right angles 

 to the direction of the other tissues, and which thus ultimately separates 

 the inert leaf from the living stem as by a knife-blade. 



Structure of the Floral Organs. Bracts, sepals, petals, &c. are 

 organized on the same plan as leaves, their epidermis frequently 

 presenting raised conical cells and having stomata. The tissues of 

 these organs are more delicate, the fibro-vascular structures being 

 almost exclusively formed of spiral vessels. The parenchyma of 

 petals contains fluid colouring-matters instead of chlorophyll. 



In the parenchyma of the floral organs of coloured structures, the cells 

 are filled with fluid colouring-matters of various tints, the depth of colour 



