de Frame .— The Seedling Structure of certain Cactaceae. 139 
change occurs, except for the slight and gradual passing in of the vascular 
elements towards the centre of the axis. In the usual case, the root 
possesses four well-marked, though small, phloem groups and only two 
bundles of xylem. This root arrangement is usually maintained so long as 
any differentiation of the tissues can be observed, but one seedling examined 
showed an interesting variation. Towards the base of the root each of the 
protoxylems sent off a branch, which finally resulted in the formation 
of a tetrarch root of the normal type (Diagram 5, Figs. 3 and 4). 
In all the seedlings of Cereus tortuosus examined, the xylem almost 
entirely consisted of protoxylem ; metaxylem tracheides appear just below 
the cotyledonary node and are situated on either side of the protoxylem 
group, between it and the phloem (Diagram 5, Fig. 2) ; occasionally 
a few elements may appear between the groups of protoxylem. The 
root, however, rarely possesses any wood except the protoxylem, in the 
stages examined. 
The metaxylem elements are very large in transverse section, and the 
thickening of the wall projects far into the cavity. They are always sharply 
defined from the small protoxylem elements (compare with Fig. 19), and 
consist of the short, broad tracheides with the annular or spiral thickening 
ridge, which are described by Solereder 1 as occurring in some species 
of the Cactaceae. According to his observations these ‘ barrel ’ tracheides 
seem to be entirely wanting in the stems of the species of Cereus , but 
however that may be, they are certainly to be found in the seedlings. Van 
Tieghem’s 2 studies of these cells led him to the conclusion that they were 
merely a peculiar form of parenchyma, but an examination of well lignified 
elements in this and in other genera has failed to reveal the presence of 
cytoplasm and nucleus. It seems probable, therefore, that as Darbishire 3 
suggested, Van Tieghem’s observations were made on very young tracheides, 
for at the stage when the thickening band is not completely lignified, 
cytoplasm is usually seen. 
Cereus peruvianus , Mill. The seedlings of this species show rather 
more variation in their cotyledonary bundles than do those of C. tortuosus. 
In one series the small cotyledon had but one lateral strand present, while 
the larger cotyledon had two ; another seedling developed no laterals, while 
a third had only a median bundle in the smaller cotyledon, but in the 
larger had two strands, slightly inclined towards one another, which during 
the transition behaved as a f double’ bundle. The transition phenomena 
are almost identical with those of C. tortuosus , with the single exception to 
be described. 
1 Solereder : Systematic Anatomy of the Dicotyledons, Vol. i. Trans, by Boodle and Fritsch. 
Oxford, 1908. 
2 Van Tieghem : Cell, annelees et spiralees des Cact. Bull. Soc. Bot. de France, 1885. 
3 Darbishire, O. : Observations on Mamillaria elongata , Ann, Bot., xviii, 1904, 
