MEDULLARY RAYS 



105 



application to other cases. Constancy in the structure of such 

 pits has been found to be characteristic of Cordaites, Gingko, the 

 Taxaceae, and all the lower forms of the Coniferae, from which 

 we may conclude that the bordered pit is essentially a primitive 

 character. On the other hand, variation is a well-marked feature 

 of the pit in the genus Pinus, as first expressed in the large oval 

 or squarish and open pits of P. resinosa or P. Thunbergii, and 

 as later appears with greater frequency in the smaller and very 

 inconstant pits of P. taeda or P. palustris. Such variations, then, 

 involving a gradual and complete transformation to the condition 

 of simple pits, are characteristic only of the more highly developed 

 pines, from which it may be concluded that it is a feature con- 

 sistent with a relatively high order of development in exact accord 

 with the principles governing parallel changes in the pits of the 

 wood tracheids. They are also in harmony with the well-known 

 principle that variation is always of a more simplified form in 

 primitive types, but that it tends to greater diversification with 

 advance in organization and general development, as a necessary 

 sequence to the adjustment of the organism to a wider and more 

 complex environment. Finally, it has been shown that the elimi- 

 nation of the bordered pit proceeds concurrently with the more 

 complete organization of the ray tracheids, in response to a sub- 

 stitution of functional activities between these structures and the 

 degenerate parenchyma cells. We may therefore conclude that 

 extreme variation in the character of the pit is an expression of a 

 higher type of development, and that from this standpoint such 

 structures have a definite value in solving questions of descent. 

 The terminal walls of the ray cells present three variants with 

 respect to secondary growth. All the more primitive Cordai- 

 tales and Coniferales are characterized by thin walls. Cupressus 

 and Juniperus are chiefly distinguished by their thin walls, which 

 are also locally thickened, a feature which has been shown to be 

 due to incipient secondary growth. But such alterations are 

 already foreshadowed in Libocedrus, where the local thickening 

 of the wall is of a sporadic nature. In Abies magnifica and A. 

 grandis there is a partial recurrence of thin and locally thickened 



