LEAF MODIFICATIONS. 



93 







Plants which grow m excessively dry or 

 desert regions, and which are thns verj' 

 liable to suffer from excessive evaporation, 

 ordinarily have their leaves modified in 

 some way so as to guard against this ten- 

 dency. They may become merely reduced 

 in size or may be otherwise modified so as 

 to reduce the amount or the degree of ac- 

 tivity of their epidermal tissue, or they 

 may disappear altogether, or become 

 transformed into organs of a different 

 cliaracter. In one of these forms the loaf 

 becomes converted into a spine, or a group 

 of spines, each consisting of one of the 

 teeth. In this condition the leaf serves an 

 important function in protecting the plant 

 against destruction by desert animals. At 

 other times the blade (Fig. 472a) entirely 

 disappears, a new blade (rhyllodium. Fig. 

 427b) of much less activity as an evapor- 

 ating organ, becoming formed by the flat- 

 tening out or expansion of the petiole (c). 

 A phyllodinm is readily distinguished from 

 a leaf blade in that its broad surfaces are 

 directed laterally instead of vertically, as 

 in the true lamina. 



Leaves or their petioles frequently be- 

 come modified into floating organs in 

 aquatic plants, as in the case of the blad- 



dery-inflated petioles of the Eichornla 

 (Fig. 473a). 



Somewhat similar inflated organs exist 

 upon the petioles of some plants and 

 serve as the homes of colonies of ants, 

 which are eflicient in protecting the plant 

 against the attacks of certain animals 

 (Fig. 474a). 



The ofTice of climbing is frequently per- 

 formed by a portion of the leaf. In some 

 cases, as the Clematis (Fig. 475), the 

 petiole of the leaf becomes twining for 

 this purpose. At other times the apex of 

 the rhachis (Fig. 476) becomes a tendril, 

 either simple or brnuching, while at 

 others the entire leaf becomes thus modi- 

 fied. In the Smilax (Fig. 477) it is the 

 stipule w^hich is thus changed. In other 

 cases (Fig. 478) climbing is effected by 

 means of hooks developed upon some por- 

 tion of the loaf. 



Besides protecting the plant by becom- 

 ing converted into spines or spine-bearijig 

 organs, as above described, the leaf is 

 subject to various other modifications 

 having this object in view. Reference 

 has already been made to such modifica- 

 tions in the form of bud scales. For the 

 protection of the flower exists the opi- 

 calyx and such scales, called Floral 

 Leaves or Bracts, as have been described 

 in our opening account of the flowers of 

 the willow. 



Floral leaves or bracts do not always 

 exist merely for purposes of protection. 

 In very many cases they are functionally 

 a part of the flower structure, surround- 

 ing either single flowers or clusters of 

 flowers, and serving by their large size or 

 brilliant colors, or both, to attract insect- 

 visits, precisely the same as has been de- 

 scribed in reference to the perigone. 

 Through the floral bracts thus modified 

 we get a direct transformation into the 

 parts of the perigone as has already been 

 sufficiently explained. It is also import- 

 ant to note that a direct relation is to be 

 ti-aced between the definite arrangements 

 of foliage and floral leaves, as will be con- 

 sidered under Phyllotaxy, and th^ ar- 

 rangement of the parts of the flower it- 

 self. So the characteristics of praeflora- 



tiou are seen to be directly dependent 

 upon the phyllotaxy. 



