132 ELEMENTS OF PLANT ANATOMY. 
Trees growing in tropical climates, where there is little distine- 
tion of seasons and consequently no regular cessation of growth, 
lack this distinction of years’ rings. (Fig. 56.) 
While the cambium normally develops both phloem and 
xylem each year, the quantity of the latter is generally much 
greater than of the former. The phloem elements are also less 
regular in their arrangement and present little, if any, distinc- 
tion between the different years’ growths. It was on this account 
that the first explanation of the difference of the growth of spring 
and fall wood was, the pressure of the rind on the inner portion, 
which increased faster in radial diameter than the rind portion. 
This would account for the shortening of the radial diameter, 
but not for the increase of the thickness of wall, nor for the 
diminution in the number of ducts in the fall. Furthermore, it 
is now generally believed, from the results of numerous experi- 
ments, that the pressure of the rind in the fall is not, as a rule, 
greater than in the spring. While there are a number of differ- 
ent reasons offered to account for it, the reason for the difference 
between spring and fall growth is still a matter of discussion. 
We have described the stem as constantly increasing in 
diameter by the intercalation of a hollow cylinder of new tissues ; 
we must now inquire what is the effect of this on those portions 
which have been separated by this intercalary growth. It is 
evident at once that the central cylinder or pith must rémain 
entirely unaffected by the new growth, or at most, suffer a little 
contraction from the pressure of the growing cylinder. It is 
different, however, with the condition of the outer cylinder 
or primary rind, which surrounds the growing tissues. It is 
easy to see that some means must be provided for its extension 
in the same ratio as that of the inner cylinder ; otherwise it 
would be torn apart and destroyed by the increasing pressure 
from within. In stems with very limited secondary growth, the 
extension of the primary rind is often accomplished by a process 
of dilatation, by which the cells are increased in size without 
