Oenothera Ruhrinervis. 335 



which have not grown out in the form of hairs. This 

 swelhng is very shght in 0. Lamarckiana. 



It has already been stated in § 3 of this Part that 

 one of the most characteristic features of 0. ruhrinervis 

 is the brittleness of its stem. The latter as well as the 

 petioles of the leaves are very fragile and break off at 

 the merest touch. The cause of this is the weak develop- 

 ment of the hard bast. It is only biennial plants or very 

 strong annual ones that break in late autumn in the way 

 that 0. Lamarckiana does when the hard bast is torn. 



A transverse section^ of the stem of a plant about a 

 meter high, taken in August, shows the bast-fibres in a 

 discontinuous ring around the outer side of the wood 

 and inside the bark, as shown in Fig. 69 A. If we com- 

 pare such a section with a similar one of 0. Lamarckiana 

 we do not at first see any difference. In both plants the 

 sclerenchymatous strands are about equally developed. 

 But if we examine a single strand under a higher power 

 we find that in O. Lamarckiana it is better developed in 

 the radial and less so in the tangential direction than in 

 0. ruhrinervis. The most important difference however 

 lies in the thickness of the walls of the individual cells 

 which, as Figs. 69 B and C show, are about half as thick 

 in O. ruhrinervis as they are in the parent species. 



There is great difference between individuals in re- 

 spect to this character depending on whether they have 

 plenty of room to grow in, or are crowded together; or 

 whether they are sown early or late. Weak plants never 

 entirely lack the bundles, though the individual cells of the 

 bundles are fewer and more tangentially arranged. They 

 often retain these characters until they ripen their fruits. 



^ For the general anatomy of the stem see Francis Ramsay, On 

 the Stem Anatomy of Certain Onagraceae, Minnesota Botanical Stud- 

 ies, Bull. No. 9, Nov. 1896, p. 674. 



