ORGANS OF NUTRITION. 



179 



ment of them also vary in different plants, they are always uni* 

 form in those of the same species, and even in some eases, 

 throughout entire natural orders, and thus they frequently 

 supply important distinctive characters in such plants and 

 orders. Thus the Cinchonace^e is distinguished from the alUed 

 order Caprifoliacete by possessing interpetiolar stipules ; and 

 the Polygonacese from its allied orders by intrafoliaceous sti- 

 pules. 



Stipules are very rare in Monocotyledons, except the ligule 

 of Grasses be considered as analogous to them (^fiy. 351). The 

 only orders of Monocotyledons in which they undoubtedly 

 occur, are the Naiadacere and Araceas. 



The flat dilated portion at the base of many petioles, as in 

 the Umbelhferie {fiy. 258), is by some botanists regarded as 

 formed by adherent stipules ; this part is sometimes called 

 the pericladium. The fibrous sheath at the base of the leaves 

 of Palms, which is called the reticulum, is by some also thought 

 to be a stipular appendage. 



Fig. 360. 



/. ANOMALOUS FORMS OF LEAVES. 



We have already seen that the branches of a stem some- 

 times acquire an irregular development, and take the form of 

 Spines and Tendrils (see page 109). In 

 the same manner, the leaf, or its parts 

 may assume similar modifications, as well 

 as some others still more remarkable, 

 which we now proceed to describe. 



Spines of Leaves. — Any part of the leaf 

 may exhibit a spiny character owing to 

 the non-development, or diminution of 

 parenchyma, and the hardening of the 

 veins. Thus, — 1st, In the Holly {Jig. 

 302), and many Thistles (Jig. 260), the 

 veins project beyond the blade, and 

 become hard and spiny ; in some Solana, 

 the spines are situated on the surface of 

 the blade ; while in the Barberry (Jig. 

 360) the blade has little or no parenchyma 

 produced between its veins, which are 

 of a spiny character, so that the whole 

 blade becomes spinous. These spines 

 may be readily distinguished from those 

 already described which are modified 

 branches, because in the latter case they 

 always arise from the axil of the leaf, in- 

 stead of from the leaf itself. Spines may 

 also be readily distinguished from prickles 

 by theii- internal structure and other 

 n2 



Fig. 360. A portion of a 

 branch of the Barberry 

 ( Berberis vuhjaris ), 

 bearing spiny leaves. 

 The upper leaf is com- 

 posed entirely of har- 

 dened veins, without 

 any parenchyma be- 

 tween them. 



