158 MINUTE STRUCTURE OF THE LEAF. 



side veins strike off through the whole length of a strong midrib. 

 In both these cases the veins divide and subdivide and have 

 numerous cross-connections both large and small, until the ulti- 

 mate ramifications are in great part free. 



440. Thus it appears that in both types there is abundant 

 communication between the veins of leaves ; but in some cases, 

 especially in rudimentary and submerged leaves, in the leaves of 

 Coniferae, etc., the veins are very generally free, and few if any 

 cross-veinlets are met with. 



441. The fibre- vascular bundles of leaves are essentially like 

 those of stems (see 365), and need no special description here. 

 Their extremities are for the most part trache'ids, often arranged 

 in double rows, but their diversities of structure and arrange- 

 ment are innumerable. One of the more striking special cases of 

 these has been already shown in the illustration of a water-pore 

 (v, Fig. 55) ; others will be considered later (see "Insectivorous 

 Plants"). The trache'ids which terminate the final ramifications 

 of the veins in leaves are in close contact with parenchyma cells. 



442. According to Casimir De Candolle, the leaf ma}- be re- 

 garded histologically as a branch with its upper, that is its 

 posterior, side atrophied. 1 



443. The stipules have the same arrangement of elements in 

 their fibro-vascular bundles as the blade, that is, liber below 

 (outside), wood above (inside). But in ligules (organs which are 

 formed by radial deduplication) the arrangement is just the 

 reverse of this, the liber is above, the wood below. 



444. Parenchyma. The forms of the parenchyma cells which 

 constitute the pulp of leaves are : (1) spherical or nearly so ; 

 (2) ellipsoidal, sometimes much elongated ; (3) branched, some- 

 times stellate. Examples of these three are often met with in 

 the structure of a single leaf; the upper layers generally being 

 composed of ellipsoidal cells, the lower layers of more nearly 

 spherical ones, intermingled with some which are branched. 



445. The arrangement of the parenchyma of the leaf-blade 

 is referred by de Bary 2 to two chief types : (1) the centric, in 

 which the chlorophyll parenchyma is uniformly disposed through- 

 out the whole organ ; (2) the bifacial, in which there is a de- 

 cided difference between the compact tissue of the upper and the 

 spongy tissue of the lower side of the leaf. 



' Archives des sciences de la Bibliotheque universelle, 1868, tome xxxii. 

 p. 32, "un rameau a face posterieure atrophied. " 

 2 Vergleichende Anatomic, p. 423. 



