114 ; BOTANICAL GAZETTE [AUGUST a 
with the ray tracheid is then nearer the cambium, and so what we — 
really have is the replacement of a row of tracheary elements by 
parenchymatous ones. The origin of such a row of ray tracheids is 4 
to be looked for toward the pith, and, as I have shown above, the ray _ 
tracheids arise here either in connection with transitional tracheids, — 
or, when these are omitted, above or below the ray and not in line — 
with parenchyma cells. : 
The intimate study of the medullary ray from its beginning at 
the pith in the different regions of the individual plant, besides dis- 
closing the origin of the various types of ray tracheid, has drawn 
attention to important features in the distribution of these structures. 
Kwny has called attention to their absence in the first year’s growth a 
of the stem, and Jerrrry and CHRYSLER state that they are not present a 
in the seed cone. In the young branch of Pinus Strobus they appeal 
sporadically during the second and third years, and then increase — 
slowly in number until the adult condition is attained after about ten 
years. In the seedling they are still later in appearing, as is also the 
case in the young root. In the latter region they never become SO 
numerous as in the stem. In Pinus resinosa they appear in abun- 
dance much earlier and increase in number much more rapidly, reach- 
ing the adult condition in five or six years. They are never found in 2 
the cone axis of either species. ‘Thus in the primitive regions of the a 
plant ray tracheids do not occur, and therefore they must be regarded 
as specializations. That they are of cenogenetic origin is further indi- 
cated by the fact that in the older pines, the Pityoxyla of the Creta- 
ceous, as described by JeFrrey and Curys.er (6), no ray tracheids 
occur. 
In view of their origin and distribution, ray tracheids are regarded 
as specialized structures and their phylogenetic meaning so inter 
preted. Those woods in which they are most abundant are co? 
sidered most modern, unless, as in Abies, it can be shown that they 
have been secondarily lost. Their character when present is 4 
important as their number. For example, if the early growth show? 
many transitional elements or a large development of tail-like pro” 
jections, then the wood is stamped as primitive. Again, the smooth- 
wall form in the soft pine is more tracheid-like and therefore MOre 
primitive than the dentate form of the hard pine, an inference which 
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