GENERAL LAWS OF ANIMAL STRUCTURE. 245 



acacias is not homologically a leaf at all, but a leaf- 

 stem, broadened and flattened to perform the function 

 of a leaf. Such an organ, functionally a leaf but homolo- 

 gous with a leaf-stem, is called by botanists a phyllode. 

 Its true nature is shown in the development of a simple- 



Fig 157 —A branch of young acacia, showing change from one form of 

 leaf to the other : <z, 6, c, d, successive stages of change ; Is, leaf stalk 

 which gradually changes into the blade in c, d, and e. 



leaved acacia from a seedling. At first it bears only 

 feather leaves, then imperfect feather leaves with flat- 

 tened stems, then broadly flattened stems with a mere 

 remnant of featherlike leaflets, and finally the leaflets 

 are gone and only the broad leaf-stems remain. All these 

 forms may often be seen in the same spray (Fig. 157). 



