2 19 
Herbaceous Type in the Angiosperms. 
of cells are arranged in radial sheets like normal ray cells. In the genera 
and species mentioned above various stages intermediate between the 
typical ray cell and the condition found in A. striata occur ; the change of 
position of the long axis from horizontal to vertical is very gradual, the cells 
apparently becoming square as a first step. 
In the genus Potentilla we have a considerable range from the shrubby 
P. fruticosa, L., to the annual or biennial P. monspeliensis , L. But the 
reduction of the central cylinder has not been extreme ; in the aerial stems 
of these plants separate bundles occur, so far as the investigations of the 
writer have gone (in fifteen species), only in the stolons of P. Anserina , and 
in the erect stems of P. palustris. However, the rootstocks of several 
species — for example, P. monspeliensis , L., P. pumila , Poir., P. intermedia , 
L., and P. pennsylvanica , L. — especially show a dissection of the central 
cylinder into alternating segments of xylem and parenchyma. These paren- 
chymatous segments, however, are not entirely transformed, for each has 
a small group of xylem elements on its medullary border. This type 
of structure is somewhat similar to that of the rhizomes of Sanguisorba and 
Genm , mentioned above. The explanation of such a condition is best 
obtained from the seedling. Serial sections of the seedling stem show the 
small centrad groups of xylem which subtend the large segments of paren- 
chyma to be leaf-traces. As in Quercus , a segment of the central cylinder 
is set off for some distance below the passing out of a leaf-trace by the 
formation of large rays resulting from xylem parenchymatization. A pro- 
gressive change of the xylem of this segment into parenchyma occurs 
upwardly towards the point of exit of the trace ; more and more of the 
secondary xylem becoming transformed until only the primary tissue is 
left. This now subtends pure parenchyma, through which it passes out 
as the leaf-trace. In the older plant with its crowded leaves these seg- 
ments, which are somewhat longitudinally extended, overlap, giving the 
appearance shown in Fig. 5. That these parenchymatous segments in 
reality represent xylem-transformations is clear, not only because all stages 
of development can be followed, but because scattered groups of lignified 
elements and solitary vessels or tracheides often appear in them. All that 
is necessary to produce a typical herbaceous stem from this structure is to 
reduce the size of the normally lignified xylem segments ; many semi- 
herbaceous forms have such a reduced xylem system. In a stem with 
a thin xylem ring, the setting off of the leaf-traces alone is sufficient to 
form the discrete bundle system of the herbaceous type. 
That such is definitely the case sometimes is exemplified by two cases 
among the plants examined. For a short distance above the base the 
erect, fertile branches of Potentilla palustris and Geum rivale have a con- 
tinuous woody cylinder. This becomes broken up into bundles very 
quickly — within a centimetre or less from the rhizome. Serial sections 
