142 THE ANATOMY OF WOODY PLANTS 



general external form of the foliar structure may reveal the presence 

 of a narrow base or stalk known as the petiole, and this is termi- 

 nated by the broad, generally horizontally placed, region called the 

 blade or lamina. The petiole often has related to it paired and 

 lateral appendages known as stipules. Occasionally an unpaired 

 appendage of the nature of a stipule, often called the ligule, 

 is present. 



THE SPORANGIUM 



This is the organ of reproduction. Although ordinary vegeta- 

 tive parts under certain conditions, and particularly in herbaceous 

 forms, may serve to perpetuate the plant, still the sporangium is the 

 special organ of reproduction and has the most important evolu- 

 tionary significance in this respect. The organ under discussion 

 is very accurately defined by the fact that it produces spores and 

 is, indeed, the only structure in the higher plants set apart for this 

 purpose. In some cases the essential fact of sporogeny is more 

 or less concealed as the result of the deep-seated modifications con- 

 nected with heterospory and the seed habit; but in such instances 

 a study of internal organization suffices to make the situation clear. 

 Normally the sporangium is a structure which is an appendage 

 of the leaf. Sometimes it appears to be axillary, although there 

 is no reason to believe that this was its primitive relation to 

 the foliar organs. Very frequently the reproductive leaf undergoes 

 profound modification in connection with the functions it has to 

 perform, and these features furnish a generally approved basis of 

 systematic grouping for the vascular plants. They need not be 

 considered in any detail in an elementary work on anatomy and are 

 best discussed in connection with particular groups in appropriate 

 later chapters. Unlike the other organs, the sporangium shows 

 no indications of derivation from another structure. It has, in fact, 

 been suggested that the sporogonium of the lower bryophytes is 

 the ancestral primordial from which all the other organs have been 

 derived. Professor Bower has developed this idea in his Origin of a 

 Land Flora. 



