78 



a bushel of leaves picked from a tree in autumn are noticeably 

 heavier than the ashes from a bushel of leaves picked fmm 

 the same tree in spring. The growth of the stem and the 



FIG. 66. Cells from the rind of ail orange, showing the colored chromoplasts 



production of new leaves which shade the older ones make 

 it desirable to cut off the latter after a time. The fall of the 

 leaf is caused by a layer of brittle cells which the plant con- 

 structs across the petiole. When the 

 parts are cast off, a smooth scar is 

 left, over which a thin cover of bark 

 is deposited. Great numbers of flow- 

 ers and embryo fruits are cut off 

 by the plants in the same way, and 

 many woody species also cut off some 

 of their twigs. The latter are usually 



the young twigs of the season and of 



FIG. 67. Typical flower of 



the houseleek 

 pet, petals; sta, stamens; car, the same age as the leaves. 



carpels; sep, sepals _ TT1 



The flower. When the season lor 



reproduction arrives the flowers appear. These may be re- 

 garded as transformed branches designed for reproduction. 

 The flower when complete has four sets of organs, called 



