ORGANS OF NUTEITION. 



173 



petiolar stipules ; and the plants of the Polygonacese from those 

 of allied orders by intrafoliaceoiis stipules. 



Stipules are very rare in Monocotyledons, except the ligule 

 of Grasses be considered as analogous to them. The only orders 

 of Monocotyledons in which they undoubtedly occvir are the 

 Naiadacese and Araceae. 



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

 the Umbelliferse {fig. 255), is by some botanists regarded as 

 formed by adherent stipules ; this part is sometimes called the 

 pericladhim. 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. 



7. AN03IAX0US FOEMS OF LEAVES. 



We have already seen that the branches of a stem sometimes 

 acquire an irregular development, and take the form of Spines 

 and Tendrils (see page 103). In the same manner the parts of 

 a leaf 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 paren- 

 chyma, and the hardening of the veins. Thus, — 1st, In the 



Fig. 357. 



Fig. 358. 



Fig. 357. A portion of a branch of tbe Barberry (Berberis vldgaris^, bearing 

 spiny leaves. The upper leaf is composed entirely of hardened veins, 



without any parenchyma between them. Fig. 358. A portion of a 



branch of the Gooseberry (^Ribes Grossidaria). f, f. Scars of leaves, with 

 buds in their axils, c. Spine produced from the pulvinus. 



HoUy (fig. 297), and many Thistles ( fig. 257), the veins project 

 beyond the blade, and become hard and spiny ; in some species of 



