110 



MORPHOLOGY 



171. The diminishing leaves 

 of boneset. 



desert plants; and they may then serve a purpose in lessen- 

 ing transpiration. It is a common notion that spines and 

 prickles exist for the purpose of keep- 

 ing enemies away, and that hairs keep 

 the plant warm, but these ideas usu- 

 ally lack scientific accuracy. Even if 

 spines do keep away browsing animals 

 in any plant, it is quite another ques- 

 tion why the spines came to be. To 

 determine what spines and hairs are 

 for demands close scientific study of 

 each particular case, as does any other 

 problem. 



231. Leaves are usually smaller as 

 they approach the flowers. (Fig. 171.) 

 They often become so much reduced 

 as to be mere scales, losing their office as foliage. In their 

 axils, however, the flower-branches may be borne. (Fig. 172.) 



Much-reduced leaves, particu- 

 larly those that are no longer 

 green and working members, 

 are called bracts. In some 

 cases, large colored bracts are 

 borne just beneath the flowers 

 and look like petals: the flow- 

 ering dogwood is an example; 

 also the bougainvillea, which is 

 common in glasshouses ; also the 

 scarlet sage of gardens, some of 

 the euphorbias or spurges, and 

 the flaming poinsettia of green- 

 houses. Sometimes a green leaf 

 is borne close against a head or 

 cluster of flowers, as in the clover (Fig. 173) ; but a separate 

 bract or scale will be found for each flower in the head. 



172. The uppermost flowers are borne 

 in the axils of bracts. Fuchsia. 



