INVOLUCRE. 



9-3 



nature is certainly obscure. Tliey are subject to the same laws of venation and 

 form, perform the same functions, and are sometimes almost undistinguishable 

 from the leaves themselves. They also (very rarely) develop buds in their axils. 

 a. When they grow from the stem itself, they may, therefore, be regarded as 

 rudimentary haves, but when from the base of the petiole, as is most common, 

 they are the undeveloped leaflets of a pinnate leaf, as in the I'ose. 



251. When leaves are furnished with stipules they are said 

 to be stvpulale, and when without them they are exstipulate. 

 Tlie stipules which are situated at the base of leaflets are called 

 siipcls. 



FIG. 35. — Stipules, Bracts, &c. 1, a, stipule of grass ; 2, 6, of rose ; .3, c, bract of Tilia ; 

 4, d, of a Campanula ; 5, Slum, a, involucre, c, iuvolucel ; G, Cornus Canadensis, a, colored 

 involucre, c, flowers ; 7, Arum, a spathe, c, spadix. 



252. Bracts, called also fioral leaves, are leaf-like append- 

 ages, intermediate between leaves and the floral organs. From 

 leaves they are generally distinguished by their being placed 

 near the flower, their smaller size, their difference in form, and 

 often in color. 



253. That bracts are of the same nature as leaves is perfectly evident, for so 

 ^•adtial is the transition between them tliat no absolute limits can be assigned. 

 That tliey have a common oj-igin with the sepals of the cah'x also, is equally evi- 

 dent, — so imperceptibly do the latter pass into bracts; affording one of the 

 strongest proofs of the doctrine of floral metamorphosis. 



a. Bracts have received different names, according to tlieir arrangement and 

 situation. They constitute an 



254. Involucre, when they are arranged in a whorl, and snr 

 round several flowers. In the Phlox, and generally, it is green, 



