2l6 
Eames . — On the Origin of the 
lating that of an herbaceous dicotyledon has been produced. Evidence has 
been brought forward 1 in proof that the bundles of this stem correspond to 
the primary structure of the Calamite stem. Such reduction appears highly 
significant. 
Among the Cryptogams these changes seem to have produced more 
resistant plants — forms better fitted for the struggle, with new plant-groups 
and with new conditions. Likewise among the Angiosperms, the latest 
dominant vegetation, the herbs are the more adaptable and more efficient 
forms. Maturity is reached very early, with the least possible expenditure 
of material for vegetative structures ; and, further, a larger amount of seed, 
in proportion to the size of the individual and the space it requires, is pro- 
duced. Evolutionary progress can be rapid among such forms. Among 
herbaceous plants the annual would then be the type of highest development. 
It is significant that those members of the Dicotyledons now generally con- 
sidered the lowest are largely trees and shrubs, and that most of the Com- 
positae outside the tropics are herbaceous. The vast majority of the 
Monocotyledons are herbs. Proof that the anatomical structure of the stem 
of the herbaceous Dicotyledons is the highest type within that group will 
add further evidence to that which leads to the opinion that the Mono- 
cotyledons are the more recent group of the Angiosperms. 
If, from the solid cylinder, the stem with a ring of separate bundles has 
been evolved by reduction and dissection, it may be asked what factor has 
caused the change, or given the impulse that brought about so great 
a transformation. The process is to be attributed to the leaf-trace and its 
influence. It has been accomplished by the local transformation of secon- 
dary xylem into parenchyma below and above the entering leaf-trace ; by 
the elimination of segments of the protoxylem ; by the loss of the inter- 
fascicular cambium ; and, finally, in some Dicotyledons and J in the Mono- 
cotyledons, by the disappearance of the fascicular cambium also. Along 
with the elaboration of interfascicular parenchyma by the transformation of 
secondary wood — essentially a development of huge rays — goes, pari passu , 
the gradual elimination of radial parenchyma within the xylem segments 
thus set off. The bundles of the Monocotyledons universally, and those of 
many herbaceous Dicotyledons, are thus entirely destitute of rays. This 
loss of rays is very gradual, and in some instances, at any rate, takes place 
by the transformation of the ray cells into a vertical parenchyma system, 
the elements gradually changing shape, so that the long axis, at first hori- 
zontal, ultimately becomes vertical. As a final step, these ray-like series 
break up into aggregate, or solitary, rows of wood-parenchyma cells, the 
ray form thus disappearing. 
The investigations made by the writer on these points have so far been 
1 Eames, A. J. : On the Occurrence of Centripetal Xylem in Equisetum. Ann. Bot., vol. xxiii, 
1909. 
