5 
Ray Tracheids in Sequoia sempervirens. 
parenchyma cell in line with it, a considerable distance to the left 
(the intervening portion has been omitted in the drawing). This 
condition was quite frequently observed. From the above des¬ 
cription it will be seen that in Sequoia the replacement of ray 
tracheids by parenchyma is a very indefinite process, in contrast to 
the pines, nor do the cells follow one another continuously as in 
the latter group. 
In the pines, where replacement is so definite, Professor 
Penhallow, and later Thompson, considered it indicative of tran¬ 
sition between the elements, the latter clearly demonstrating that 
the tracheary element is the ancestral one. The indications in the 
transitional structures themselves are much better in S. sempervirens 
than in the pines, simple and bordered pits occurring on the same 
element. These are present on the horizontal walls (see Fig. 2): 
at the extreme left of the marginal cell is a bordered pit, then one 
with a less distinct border, and so on towards the right where there 
is an almost perfect simple pit, which is larger than any of the 
others. That the simple pits are really simple and not bordered is 
very clear, since the opening is the same width from top to bottom, 
Fig, 7. Photograph of transverse section of the wood of Sequoia semper- 
virens, showing portions of four annual rings, x 37 - 5. 
