59i 
Struchire of Acer Pseud op I at amis . 
hypocotyl, though they may move laterally as a whole to join the adjacent 
cotyledonary strands, and subsequent to such displacement it is difficult to 
see how the intercotyledonary root protoxylems which appear below the 
external collet can be referred to them. Further, when, as in some of the 
abnormal types described in Group i, the epicotyledonary midribs persist 
below their normal level, they appear to delay the formation of the inter¬ 
cotyledonary poles rather than assist in their development. It should 
also be noted in this connexion, although we do not wish to stress the point, 
that in the abnormal seedling f although only one epicotyledonary leaf is 
present, two root poles are formed in the positions which would normally be 
occupied by the intercotyledonary poles. 
The second criticism of Chauveaud’s theory was originally put forward 
by Compton ( 5 ), who pointed out that the sequence of alternate, intermediate, 
and superposed phases is not realized in types such as Lnpinus hirsutus , in 
which, at the base of the cotyledon, an isolated median protoxylem is 
present, flanked on either side by a widely separated collateral bundle, from 
which it may be separated by as many as fifteen parenchymatous cells. It 
is equally evident that this sequence is not realized in the sycamore. If the 
upper part of the hypocotyl is examined in the very young seedling (Fig. 6) 
it is found that the formation of the central protoxylem is followed by the 
development of metaxylem elements in the diagonal plane, and that a series, 
often exceeding twenty, of parenchymatous cells intervenes between the 
protoxylem and the metaxylem. It requires a good deal of faith to believe 
that this parenchymatous gap represents a region occupied ancestrally by 
intermediate xylem elements, and the facts seem to be met better by 
a different explanation. 
We may say, generally speaking, that in each seed-leaf of a dicotyle¬ 
donous seedling the vascular components consist of a midrib and a lateral 
system (Fig. 68). These may react on one another in various ways. Thus 
there may be a median concentration of vascular components leading to their 
aggregation in the cotyledonary plane so that two compound strands enter 
the hypocotyl (Fig. 69 a , b) ; alternatively the midrib and laterals may 
remain independent so that six simple strands enter the hypocotyl, two in 
the cotyledonary plane and four in the diagonal planes (Fig. 70 a, b). 
Finally, there may be a lateral concentration of the vascular components so 
that the metaxylem and phloem of the midrib are detached from the central 
protoxylem, each half moving outwards and frequently uniting with its 
adjacent lateral. In this event foJir compound strands will enter the hypo¬ 
cotyl in the diagonal planes, whilst the cotyledonary plane will show two 
isolated protoxylems (Fig. 71 a,b). Any of these three types of grouping 
may produce either a diarch or a tetrarch condition in the root, so that there 
does not appear to be any close connexion between the number of root poles 
and the type of vascular arrangement found in the hypocotyl. 
