4 THE AMERICAN BOTANIST 



than three days with a hfe-span thrice that of their compan- 

 ions in the garden. Lack of time prevented experiments to 

 decide whether heat or Hght has the greater effect on their 

 closing, hut from the fact that, late in the summer, the flowers 

 do not close during the day and are no longer true evening 

 primroses, it may be inferred that temperature rather than 

 light may be the cause. Though there are times when the 

 flowers remain open in the light, they are nevertheless true to 

 their first instincts in opening first at dusk. 



The evening primroses are all very much alike in cul- 

 tural requirements. They are dry ground plants, often very 

 abundant in desert regions, but almost any garden soil will 

 suit them. Though often blooming at night, they prefer 

 sunny situations. Our commonest species (Oenothera biennis) 

 is a frequent weed in gardens but some forms of it are suf- 

 ficiently attractive to be cultivated, and occasionally they are 

 fragrant. This species, or a closely related form, is of more 

 than ordinary interest because of the fact that from it De Vries 

 obtained some of the best proofs of his Mutation Theory 

 which in some respects modifies the well-known Darwinian 

 Theory of the origin of species. The particular form he 

 worked with is known as O. Lamarkiana, or occasionally it 

 is called the variety grandiflora of O. biennis. 



The finest day blooming species of the group is proba- 

 bly O. spcciosa, a plant with large satiny-white flowers. The 

 best vellow sort is said to be O. Missoiiricnsis a trailer with 

 flowers often five inches across. The yellow-flowered form 

 oftenest cultivated in the gardens is 0. fntficosa \ariety 

 Youngii, which is usually called O. Voiingii in tlie catalogues. 

 Another white-flowered day-blooming form is O. pallida. 

 This opens in the early morning and late in the da}- turns to a 

 deej) pink. Best of all it is a perrennial and in time forms 



