696 
Thomas . — Seedling Anatomy of 
been exhaustively treated by Chauveaud 1 in his 1911 paper, which brings 
together his work on seedling anatomy, and it forms the pivot of his thesis 
on the transitional phenomena. Inasmuch as at an early age exarch or, 
at any rate, c alterne ’ protoxylem elements are present throughout the 
whole seedling, transition, according to this author, is an appearance 
largely due to subsequent obliteration of these elements in the upper 
part. 
The British work of the last ten years, following upon the presentment 
by Miss Sargant 2 of a theory of the origin of Monocotyledons founded 
upon seedling anatomy, has all had a more or less well-marked phylogenetic 
aim. In marked contrast to this is the attitude of the earlier anatomists. 
Thus, in accordance with his bundle concept, de Bary, in the classic ‘ Com- 
parative Anatomy of Phanerogams and Ferns’, treats the hypocotyl merely 
as the basal internode in which the bundles of the lowest leaves, viz. the 
cotyledons, fuse and pectinate in a, definite manner. Van Tieghem, on the 
other hand, is chiefly interested in the hypocotyl as the region in which 
the arrangements characteristic of the root-stele become modified so as to 
pass into the arrangements characteristic of the stem. 
Van Tieghem’s investigations led him to lay stress upon two processes, 
(a) that of the so-called ‘ rotation ’ of the protoxylem, which produces the 
change from endarchy to exarchy, or vice versa, according as the vascular 
strands are followed from stem to root or root to stem, and ( b ) that of the 
splitting and fusion which, together with an alteration in position, produce 
the change from collateral to radial or radial to collateral. These internal 
changes correspond to external changes in epidermal characters which 
take place in a well-defined region termed the collet , which is the plane of 
division between hypocotyl and root. Van Tieghem, however, states that 
the anatomical transition is frequently drawn out by intercalary growth of 
hypocotyl, of root, or of both. 
Rotation of protoxylem is assumed to take place in all cases, but the 
methods of change observed under ( b ) are classified in the ‘Traite de 
Botanique ’ under three types, depending upon the nature of the strands 
concerned (i. e. whether xylem or phloem, or both) in the fusions or 
splittings, and the relative number of such strands as between stem 
( = hypocotyl) and root. 
Thus in Type 1 the xylem changes position ; in Type 3 the phloem 
changes position ; while in Type 2 both do so. 
In Type 1 the number of collateral strands in the upper part of the 
hypocotyl is the same as the number of phloem groups in the root ; in 
Type 3 the number is the same as the number of phloem or xylem groups 
1 Loc. cit. 
2 Sargant, E. : A Theory of the Origin of Monocotyledons founded on the Structure of their 
Seedlings. Ann. Bot., vol. xvii, 1903. 
