92, 93.] 



MOKPHOLOGY OF THE LEAF. 



109 



as in the Pea and Pansy. In these cases and others 

 they act the part of leaves ; again they are often very 

 small and inconspicuous. 



279. An Ochrea is a membranous sheath inclosing 

 the stem from the node upward, as in the Knot-grass 

 family (Polygonacese). It is formed of the two stipules 



303, Leaf of Selinum, tripinnate, with sheathing petiole. 304, Leaf of Polygonum Pennsylvanicum, 

 with its ( o ) ochrea. 305, Culm of Grass, with joint ( j ), leaf ( I ), ligule ( ). 306, Leaf of Pear-tree, with 

 slender stipules. 



cohering by their two margins. In case the two stip- 

 ules cohere by their outer margin only, a double stip- 

 ule is formed opposite to the leaf, as in the Button- 

 wood. If they cohere by their inner margin, the 

 double stipule appears in the leaf axil, as in the Pond- 

 weed (Potamogeton). The Ligule of the Grasses is 

 generally regarded as a double axillary stipule. The 

 leaflets of compound leaves are sometimes furnished 

 with little stipules, called stipels. 



280. Inter-petiolar stipules occur in a few opposite- 

 leaved tribes, as the Galium tribe. Here we find them 

 as mere bristles in Diodia, while in Galium they look 

 like the leaves, forming whorls. Such whorls, if. com- 

 plete, will be apparently 6 -leaved, consisting of two 

 true leaves and four stipules. But the adjacent 

 stipules are often united, and the whorl becomes 

 4-leaved, and in some the whorl is 8 -leaved. 



