162 de Frciine. — The Seedling Structure of certain Cactaceae. 
Mamillaria hexacantha , Salm-Dyck. The transition-features in these 
seedlings are practically identical with those of M. eriacantha , in which 
cotyledonary bud-bundles are absent. 
Mamillaria spinosissima , Lem. The transition-phenomena of most of 
the seedlings of this species which were examined were practically identical 
with those of M. eriacantha , in which the tubercle-bundle was absent; one, 
however, showed a somewhat different method of procedure. In this seed¬ 
ling the upper hypocotylar region possessed six epicotyledon-traces lying 
in two groups of three, in alternation with the cotyledonary bundles. At 
a lower level each of the two groups fused into one strand, which, unlike 
what usually obtains, remained in position during the remainder of the 
transition. Concurrently with the fusion of the three bundles, the cotyle¬ 
donary phloem bifurcated; the two halves thus produced rotated and at the 
same time moved gradually inwards ; thus towards the middle region of the 
hypocotyl the epicotyledonary bundles each showed a phloem group derived 
from the cotyledon-bundles lying on either side of it; these phloem groups 
soon fuse laterally with epicotyledonary phloem ; the epicotyledonary 
xylem dies out, but this is probably to be attributed to the age of the seed¬ 
lings. During these changes the protoxylems of the cotyledon-strands, 
which by the movement of the phloems had been left exposed, undergo 
centripetal displacement ; finally large metaxylem elements bridge the space 
between them and the diarch root is completed. 
Mamillaria Donatii , Berge. The transition-phenomena in these 
seedlings very closely resemble those which are found in the seedlings 
of M. pentacantha which possess no cotyledonary tubercle-trace. The 
bundles are, however, very small, and the xylem is composed of very few 
elements. 
Mamillaria missouriensis, Sweet. The seedlings of this species differ 
in one respect from all of the above described Mamillarias. The papillate 
cotyledons possess at first one median vascular strand, which almost 
immediately branches into three. In one of the two seedlings available for 
examination two of these strands (£ and c y and d and e y Diagram 18, Fig. i) 
fused in the upper part of the hypocotyl, so that there were present four 
cotyledonary and four epicotyledonary bundles. 
The pairs of cotyledonary strands rotated towards each other as is 
shown in Diagram 18, Fig. 2, and at a lower level one epicotyledonary bundle 
(e) fused with each seed-leaf-trace (Diagram 18, Fig. 3), but this fusion does 
not take place simultaneously. After it has been accomplished the four 
phloem-strands separate ; accompanied by the metaxylem tracheides, which 
from this stage gradually die out, they finally take up a position such as is 
shown in Diagram 18, Fig. 4. Towards the base of the hypocotyl each of the 
protoxylems branch, and as a result a tetrarch root is produced (Diagram 
18, Figs. 5 and 6). 
