142 ' Introduction to Botany. 



been unable to produce variations from which a selection 

 of suitable forms could be made. The better-nourished 

 stems, however, would have been able to do this, and 

 finally the forms which we now see could have been 

 evolved. This description is of course conjectural, and 

 may, or may not, approximate the true events. 



113. Modified Leaves. Leaves modified for storage are 

 found in the scales of the bulbs of onion, in the thicker 

 scales of tiger lily bulbs, and in the leaves constituting the 

 cabbage head. The fleshy leaves of Agave Americana are 

 storehouses of water and reserve food, while at the same 

 time they carry on the normal constructive functions of 

 ordinary leaves. The thick leaves of succulent plants, 

 such as Mesembryanthemum and Sedum, serve as store- 

 houses for water. Leaves which have been modified to 

 serve a protective function are seen in bud scales, and in 

 the spines of cacti, and in barberry. Parts of leaves which 

 have been modified to form tendrils for climbing, we find 

 in Solatium jasminoides, the garden pea, etc. 



We have already, in the last chapter, become acquainted 

 with leaves which have motor organs, digestive glands, 

 sensitive hairs, etc., for the capture of insects. There are 

 other modified forms of leaves which entrap insects and 

 apparently use them for food. These are the pitchers of 

 the pitcher plants and allied forms, and the bladderlike 

 traps of the bladderwort. In these cases, the modifications 

 are so great and their adaptation as traps is so wonderful 

 that they deserve a somewhat detailed description. 



114. Pitcher Plants. The pitcher plants (Nepenthes) 

 are natives of the old world tropics. The pitcher is borne 

 at the end of a slender prolongation of the petiole, and is 

 probably itself a part of the petiole which has grown out 

 in the enlarged tubular or pitcher form. At the top of the 



