‚530 THE AMERICAN NATURALIST. . (Vor. XXXIX. 
only a few elements broad, and there is an abrupt transition 
from spiral or scalariform structures to the tracheids with bor- 
dered pits. We may therefore consider that in all the higher 
Coniferales, the structural type is fixed to such an extent that 
evolutionary aspects are no longer presented. 
The changes initiated in the transition zone relate to the evo- 
lution of the modified vessels or tracheids which constitute the 
bulk of the structure in the vascular cylinder. In the develop- 
ment of such tracheids their capacity for the movement of the 
transpiration current is not lost, except in special cases, but it is 
rather modified with reference to the total extent of the conduc- 
tive tissue and the somewhat low rate at which the transpiration 
current is required to move. Each tracheid is provided, among 
the primitive forms, with bordered pits on both the radial and 
tangential walls through which a somewhat ready and direct 
transverse diffusion is provided for, while diffusion in a longi- 
tudinal direction can never be otherwise than indirect and there- 
fore at a relatively slow rate. With an advance in organization, 
the bordered pits tend to disappear from the tangential walls 
where they survive only at the ends of tracheids or in the sum- 
mer wood, the latter distribution being necessitated by the 
peculiar radial compression which such cells experience. It 
follows from this that in all the higher types of the gymno- 
sperms, there is a greater degree of restriction imposed upon 
the transverse diffusion and through the latter, also a further 
restriction of the longitudinal diffusion. Whatever deficiency in 
circulation is developed through these structural alterations, 
compensation is provided by the great extent to which such tis- 
sue is formed ; and that the tracheids effectively discharge the 
duty imposed upon them in this respect is manifested in their 
complete replacement of the true vessels of the protoxylem, as 
well as in the fact that those of the heart wood may resume 
their functional capacity for the conveyance of nutrient fluids 
when the sap wood becomes incapacitated through desiccation 
or other causes (Goodwin, '88). Such restricted movement of 
the transpiration current is in direct accord with the generally 
xerophilous character of the gymnosperms, whether this latter 
' be correlated with absolute deficiency of water supplies or a 
