146 THE TEACHING BOTANIST 



in this way. The real explanation doubtless is, that, 

 although the placenta did thus originate, it has attained 

 to independent dignity as a morphological element, and 

 proceeds to act as a unit, varying and adapting itself 

 to its conditions, largely independently of its original 

 nature. Of course there are all degrees of this inde- 

 pendence of original nature ; and while some structures 

 have broken away entirely from it, others are more or 

 less bound by it ; but the recognition of the principle, 

 really the fundamental principle of modern morphol- 

 ogy, is very important. Formalism in morphology, 

 based upon comparative anatomy, must be modified by 

 realism based upon embryology. Unfortunately there 

 is as yet no authoritative work in English treating fully 

 this subject. 



Perhaps the greatest of all current morphological 

 errors is that which attempts to homologize ovules and 

 pollen with something on the green shoot. It is often 

 taught that the ovule represents an altered piece of the 

 edge, perhaps a tooth, of the leaf, while the pollen is the 

 parenchyma with epidermis rounded out to form the 

 anther. It is, however, now known beyond any doubt 

 that the ovule (strictly, its nucellus) is a spore-case, a 

 lineal descendant of the spore-cases of liverworts, and 

 hence is much older than the differentiation of the parts 

 of the leafy shoot ; and the same is true of the pollen and 

 anther, which represent also ancient spores and spore- 

 cases. Hence the pollen and ovules are not modified 



