iQisl DE VRIES— OENOTHERA BIENNIS 171 



The endemics are rare, often strictly local, and grow in the midst 

 of a luxuriant vegetation of their widely spread and thriving 

 ancestors. It is hardly necessary to point out that this conclusion 

 holds good not only for Ceylon, but for the origin of endemic and 

 local species in general. 



Willis has also called attention to the Podostentaceae and 

 the allied group Tristichaceae. They show one of the most inter- 

 esting illustrations of a very rich differentiation without the least 

 indication of a relation to their environment. A very great uni- 

 formity in the conditions of life is combined with a most remarkable 

 variety in their morphological structure. In the Podostemaceae 

 the flowers are anemophilous, terminal, and erect, but combine 

 with these characters of low organization the highest degrees of 

 dorsiventrality and of differentiation, and this without any refer- 

 ence to advantages or disadvantages to be derived from them in 

 their functions. Numerous points of similar significance in the 

 structure of the vegetative and reproductive organs are pointed 

 out by the author. Moreover, the genera Tristicha and Podo- 

 stemon, which are widely distributed, are comparatively little 

 modifiied from the earlier types of the orders, while the highly 

 specialized forms are at the same time the rarest, exactly as in 

 the case of the endemics of Ceylon.' 



In the group of the evening primroses the same principles 

 prevail. Their struggle for existence is hmited by the difficulties 

 which they have in producing roots. Cuttings almost never suc- 

 ceed in rooting, with the exception of the lateral rosettes at the 

 base of the stem. Artificial transplanting becomes difiicult as 

 soon as the main root increases in size. In the field only a small 

 percentage of the seeds germinate and thrive, and this only under 

 special conditions. They want a stirred up soil and do not like 

 to grow between other plants. These characters are common to 

 all the forms which I have had an opportunity of studying in their 

 native habitats. On the other hand, the mmierous small specific 

 differentiations, such as the form of the leaves, the branching of 

 the stem, or the structure of the flowers and fruits, do not show 



3 Willis, J. C, On the lack of adaptation in the Tristichaceae and Podostemaceae. 

 Proc. Roy. Soc. 8:532-550. 1914. 



