CHAPTER VII 



MEDULLARY RAYS (continued} 

 RELATIONS TO DEVELOPMENT 



We are now in a position to determine the relations in which 

 the various structural features of the medullary ray stand to 

 development, and for this purpose it may be most convenient 

 to discuss them in that sequence which is apparently consonant 

 with the general order of evolution of the entire group. 



It has been ascertained that bordered pits are characteristic 

 features of the lateral walls of the ray cell in 72.4 per cent of the 

 investigated species, and that in the remaining 2 7. 6 per cent, among 

 the higher types, simple pits predominate, but a closer scrutiny 

 of this latter group discloses some features of more than passing 

 interest. Reference to the table of anatomical data (Appendix A) 

 will show that the change from bordered to simple pits is entirely 

 confined to the genus Pinus, and that it does not arise abruptly 

 as if in response to some unusual condition whereby a profound 

 alteration in the usual course of development was induced ; but 

 it is effected by stages, showing that whatever influences were 

 brought to bear, they operated gradually through a somewhat 

 prolonged period of development, while here and there strong 

 tendencies to reversion were manifested, and that the alteration 

 was finally effected in a permanent way, only in the most highly 

 developed pines. Commencing with P. Lambertiana, it will be 

 observed that some species of the soft pines are characterized by 

 simple pits. Among the hard pines P. clausa and P. rigida have 

 bordered pits, while the six following species again show simple 

 pits. We next come to a group of four species, with one excep- 

 tion (P. Murrayana) Japanese, in which there is a mingling of 

 both bordered and simple pits, showing a decided persistency 



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