208 
PACIFIC SCIENCE, VoL XIII, April, 1959 
wichense and A. virescens. In Wilkesia, the 
layer beneath the epidermis is differentiated 
into a discrete hypodermis consisting of thick 
walled sclereids, and cork formation is initi- 
ated in a single layer immediately interior to 
the hypodermis. 
The stems of both Ar^roxtphium and 
Wilkesia lack endodermis demarcated by Cas- 
parian strips, although the formation of tan- 
gential sheets of cytoplasm in the layer of 
cells immediately exterior to bundle caps 
indicates that this layer of cells may be some- 
what differentiated as an endodermis. 
Biseriate glandular hairs with capitate mul- 
ticellular heads are characteristically present 
on the stems of both Arg^roxiphum and 
Wilkesia. These trichomes are of the same 
type as those illustrated by Carlquist (1957^/) 
for inflorescence bracts of Wilkesia and Arg^- 
roxiphimn sandivichense . 
The similarities between stem structure of 
Arg^roxiphium and Wilkesia and that of Du- 
hautia are considerable, and are what one 
would expect if the three genera represent 
derivations from a common ancestry. 
Secondary Growth 
Aspects of secondary xylem structure in 
Dubautia, Argyroxiphium, and Wilkesia have 
been considered in an earlier paper (Carlquist, 
1958). The stem of D. platyphylla (Fig. 23) 
demonstrates alterations in phloem and cortex 
structure as a result of secondary growth. 
Abundant cork development is evident, 
above. Formation of fibers in secondary 
phloem may be seen, below. The stem illus- 
trated is young enough so that the cortex has 
not been ruptured, and characteristics of old 
cortical cells are shown. Cortical cells have 
experienced considerable tangential stretch- 
ing. A particular feature of interest is the for- 
mation of pectic warts on the surfaces of cell 
walls which face intercellular spaces (Fig. 23, 
inset). These structures have been reported 
for cortical cells of Compositae only in 
Wyethia ovata (Carlquist, 1956). They do, 
however, occur on pith cells of Argyro- 
xiphium sandwichense (Carlquist, 19^1 d). 
Root 
Roots of the genus Dubautia were not 
studied. In Argyroxiphium and Wilkesia, which 
the writer was able to grow from seed he col- 
lected in the Hawaiian Islands, anatomical 
studies were made. Adventitious roots of A. 
Caliginii were preserved in the field. Roots in 
these genera show typical dicot root struc- 
ture. In A. sandwichense, roots varied from 
diarch to pentarch, depending on the diam- 
eter of the root. The primary root of the seed- 
ling, and all of the finer roots of a mature 
plant were found to be diarch. In the ad- 
ventitious roots of A. Caliginii, which have a 
relatively larger diameter, five to seven xylem 
poles were observed. A variation of two to 
five xylem poles was observed in Wilkesia. 
In roots of A. sandwichense, there are nu- 
merous small quadrangular "endodermal” sec- 
retory canals, of the type described by Tetley 
(1925) and Williams (1954). In the adventi- 
tious roots of A. Caliginii, a pair of somewhat 
larger secretory canals occurs exterior to each 
phloem strand. These secretory canals are 
similar to those figured by Carlquist (1957o' 
116-119) ioiFitchia speciosa. Small quadrangu- 
lar secretory canals of the type found in A. 
sandwichense are characteristic of the roots of 
Wilkesia, although they may enlarge in size 
during secondary growth in the root. 
In both Argyroxiphium and Wilkesia matu- 
ration of the central core of the root is char- 
acterized by fiber development more than 
vessel development. Conversion of some cor- 
tical parenchyma cells to sclereids was ob- 
served in roots of Wilkesia which showed 
secondary growth. 
Secretory Canals: Summary 
The details of secretory canal distribution 
in the vegetative parts of the plant have been 
detailed above. The fact that secretory canals 
are characteristic of all three genera is quite 
noteworthy, however. The mode of occur- 
rence of these canals is highly reminiscent of 
the patterns in other Heliantheae, such as 
