ORGANS or NUTRITION. 



153 



Fig. 293. h. Triple-nerved 

 leaf of the common 

 Sunflower, a. Linear 

 leaf. 



ribbed OX tripli-costate, as in the common Fig. 295. 



Sunflower (fig. 295, 6); or if two such 

 rihs arise on each side of the midrib, it 

 is termed quintuple-ribbed or quintupli- 

 costate. These ribbed leaves have fre- 

 quently a great resemblance to parallel- 

 veined leaves, from which however, 

 they may be at once distingui.shed by 

 their ribs being connected by branching 

 veins. 



d. Radiated or Palmatehj-veined. — 

 Such leaves resemble the former in 

 consisting of three or more ribs which 

 are connected by branching veins; but 

 they are distinguished by the fact, that 

 those, instead of converging to the apex, 

 diverge from the base towards the circumference, as in the 

 Sycamore, CuiTant, Vine, Geranium, Melon {fig. 289), &c. 



2. Varieties of Parallel Venation. 

 The term parallel-veined is not strictly applicable in all 

 cases, for it frequently happens that the veins are radiate, but 

 from the difficulty of finding a name Avhich will comprise all the 

 modifications to which such leaves are liable, it must be under- 

 stood that Ave apply the term parallel-veined to all leaves in 

 which the main veins are more or less parallel and simply con- 

 nected by unbranched veinlets. We distinguish two A-arieties 

 of parallel venation. 



a. Straight-veined. — In these leaves the veins either proceed 

 in a parallel direction from the base to the apex, to Avhich point 

 they converge more or less (fig. 296), as in the ordinary ribbed 

 variety of reticulated leaves already noticed, and are con- 

 nected by simple unbranched veinlets; or they diverge from 

 each other towards the circumference of the leaf (Jig. 297), as 

 in the radiated-veined variety of reticulated leaves. The leaves 

 of Grasses, Lilies, and the common Flag may be taken as ex- 

 amples of the first modification ; and those of the Palms of the 

 second. 



b. Curve-veined. — In such leaves we have a prominent mid- 

 rib, which gives off from each of its sides along its Avhole 

 length other veins, which proceed in a pai'allel direction 

 towards, and lose themselves in, the margin (figs. 298 and 

 290, b). These veins are connected, as in those of the last 

 variety by unbranched veinlets. The Banana, the Plantain, 

 and allied plants, furnish us with examples of such leaves. 



Besides the above two leading varieties of venation, the 

 leaves of Perns, and those of other Acotyledonous Plants which 

 possess veins, present us with a third; thus, in these the pri- 



