156 de Fraine .— The Seedling Structure of certain Cactaceae 
Echinocactus Ottonis , Series 2, shows a seedling in which the splitting 
of the larger cotyledon has not completely taken place, for while it 
possesses three seed-leaves, there are only two tubercles. Two of the 
cotyledons subtend one tubercle, and these two may be considered to 
represent one which has split nearly to its base. The tubercle (/ x ) 
belonging to the split cotyledon is, however, supplied with two vascular 
bundles instead of the usual one (t la} t lb , Diagram 13, Fig. 1). These 
two strands pass outwards and fuse with those of the cotyledons ( c la , 
c lb , Diagram 13, Fig. 2); the two strands thus produced undergo rapid 
centripetal displacement, rotate slightly towards one another, fuse as 
regards their protoxylems, and finally together form one bundle 
(Diagram 13, Fig. 3). The second bundle is formed by the union of 
the remaining cotyledon- and tubercle-traces (c 2 , 1 2 , Diagram 13, Figs. 2-4). 
Transition. 
In Series 2 there were present in the upper part of the hypocotyl two 
‘ double ’ bundles, produced in the manner described above, and two 
epicotyledonary bundles ( e , Diagram 13, Fig. 3). The behaviour of these 
bundles in the transition is indicated in Diagram 13, Figs. 3-5. Extra¬ 
ordinarily large ‘ barrel ’ tracheides are developed in the central region of 
the axis. 
In Series 1 and Series 3 the formation of the diarch root takes place 
in a similar manner by the fusion of the opposite pairs of phloem-strands, 
but in these seedlings there is no development of epicotylar bundles. 
Mamillaria. 
The Mamillarias have reached the ultimate stage in the development 
of a succulent habit in the seedling, which, in this group, consists of a tiny 
globular structure ending in a short, thread-like root (Fig. 17). 
Cotyledons are practically absent, for even in the species in which they 
attain their maximum size they merely appear as two minute swellings at 
the apex of the spherical hypocotyl (Figs. 17 and 18). In spite of the 
extreme reduction of the cotyledons, a reduction to microscopic papillae in 
most of the species examined, a difference in size between the two can 
always be observed. In general the stem apex is very much depressed 
(Fig. 18). 
Tubercles are well developed in those members of the genus in which 
the cotyledons attain their greatest size, e. g. M. multiceps , M. rodantha> 
M. centricirrha , M. polycdra , and M. meiacantha ; in these species it is 
only possible to differentiate cotyledonary from epicotyledonary tubercles 
by the position of the former in the axils of the cotyledons. The tubercle 
