1 82 BIOLOGICAL LECTURES. 



a partial explanation of the conduct of the little New Jersey 

 Pigweed. 



Perhaps enough has been already said to convince anyone 

 that the almost infinite variety of plant forms may have some 

 fairly direct connection with their environment. The cypress 

 tree produces its needful aerating organs when grown in the 

 water, but no sign of them in dry soil. It may be a matter 

 of wonderment, however, why these characteristic organs are 

 so quickly and readily lost. They are an acquired character to 

 fill a definite want when this want no longer exists, they dis- 

 appear. Perhaps this character, which we know was acquired 

 since the Eocene period, has endured for too short a time 

 for permanent inheritance. Many other plant characteristics 

 acquired on physiological grounds, but regarding the duration 

 of which we have no record, although we have reason to 

 believe that they have been serving their purpose for an 

 immense period of time, are perfectly permanent, and inherited 

 by their offspring long after the developing cause has disap- 

 peared. We have in Australia a large number of trees which 

 have adapted themselves to a hot and dry climate by various 

 manipulations of the leaves. In some Acacias, for example, 

 the leaf-blade has entirely disappeared, and the wing growth 

 on the upper and under sides of the petiole is an apparently 

 vertical leaf. Other members of the same family make 

 the true leaf vertical by a twist of the petiole. Although 

 accomplished in two very different ways, the object in each 

 case is plainly to lessen the tranpiration. Apparently, this is 

 a much more trivial change than the acquirement of knees by 

 the Cypress. Both fulfil a physiological necessity ; but while 

 the knees of the Cypress are lost in succeeding generations 

 if the developing cause is removed, the Acacias may be 

 cultivated for generation after generation, entirely removed 

 from the climatic conditions which produced the changes, yet 

 they will be as permanently inherited as in their own native 

 land. 



Avicennia nitida, the so-called Black Mangrove of our 

 Florida and West Indies coast, of which I have already spoken, 

 belongs to the Verbena family. Normally a dry land form, it 



