FIBROVASCULAR TISSUES: RAYS 67 



consequently only a portion of their length is represented in the 

 figure. The pitting is somewhat characteristic and is ordinarily 

 marked by the large number of pores and their consequent crowded 

 and alternating arrangement. The rays cross the direction of the 

 longitudinal elements and are characterized by their thin walls, 

 which are, however, clearly pitted laterally in relation to the radial 

 walls of the tracheids. It is to be noted in passing that there are 

 no tangential pits on the walls of the longitudinal elements of the 

 wood a feature, as has been indicated in a previous chapter, very 

 generally characteristic of the woods of the gymnosperms of the 

 Paleozoic regardless of their affinities. In the transverse section 

 of the wood the rays stand out distinctly as uniseriate files of cells, 

 having their axes radially elongated. The rays are in lateral 

 communication with the tracheids by half-bordered pits. Other- 

 wise their thin walls are not characterized by the presence of pores. 

 The tracheary elements of the wood are tangentially in com- 

 munication by numerous radial pits, but their tangential walls are 

 quite free from pores, so that in these ancient gymnosperms water 

 could make progress in the trunk only in a tangential direction. 

 The tangential view of the wood shows the radial pits of the tra- 

 cheids as well as the lateral ones of the cells of the medullary rays in 

 profile view. 



After the discussion of the organization of rays in the Cordaitales 

 we are in a favorable position to understand the condition in the 

 conifers. In this connection it will be well to start with the most 

 complicated condition in living representatives of the group, since 

 the coniferous gymnosperms, as will be shown clearly in a subse- 

 quent chapter, constitute a reduction series with the more complex 

 forms at the bottom and those with simpler organization at the 

 top. Fig. 49 represents radial and tangential views of the wood of 

 the white pine (Pinus strobus). Taking first the radial view shown 

 in a, a number of important contrasts in organization to cordaitean 

 woods are to be seen. First of all as regards the tracheids, or rather 

 such part of them as is included in the field of view, it is clear that 

 they are distinguished from similar structures in the Cordaitales 

 by the smaller number of pits and the considerably larger size of 

 these. Further, the pits in face view, instead of presenting the 



