8 o 
Ethel N. Thomas. 
plane, while the phloem groups are in the inter-cotyledonary 
plane (see fig. lc). 
Such a type of bundle and mode of transition is very wide¬ 
spread through the Dicotyledonous orders, and has been described 
by Gerard, 1 Dangeard 2 , Sterckx, 3 Chauveand 4 and Hill. 5 
When the present work was extended to other parts of the 
natural system it was found that the “ double bundle ” is sometimes 
represented by two quite separate bundles (Ricinus , Quercus, etc.) 
The homology of the extreme forms is proved beyond question 
by the existence of a perfect series of intermediate cases. Further, 
the degree of approximation on the one hand towards a single 
bundle, and on the other hand towards two completely separate 
bundles, depends to some extent, in every case, upon the level in 
the cotyledons at which the bundles are examined. 
The function of these central bundles, whether they appear as 
two widely separated entities, or so closely approximated as to 
to have the appearance of a single strand, is always the same, viz., to 
form between them one pole of the root. 6 
In addition to the two central bundles or their “ double¬ 
bundle” equivalent, there is frequently present a lateral bundle on 
each side of them, so that there are four bundles in each cotyledon 
(Ricinus, Quercus, Fagus ) or the equivalent of four ( Casuarina, 
Polygonum, Linodendron, Crataegus, Acacia, Medicago, Althaea, 
etc.) ; see text-fig. 2a. 
When the lateral bundles are large and important looking, they 
penetrate into the hypocotyl and unite with the corresponding 
bundle from the opposite cotyledon to form the alternating poles 
of a tetrarch root (text-fig. 2). Here again all intermediate stages 
1 Gerard. “ Recherches sur le passage de la Racine a la Tige.” 
Annales des Sciences Naturelles, 1881. 
2 Dangeard. “ Recherches sur le mode d’union de la tige et de 
la racine,” Le Botaniste, 1889. 
3 R. Stcrckx. “ Recherches anatomiques sur l’embryon et. lcs. 
plantules dans la famille des Renonculac^es ” Mem. Soc. Roy. 
des Sci de Lifcge. 3 e S6r, tom. ii., 1899. 
* Chauveaud. “ Passage de la position alterne 4 la position 
superieure-dans le cotyledon de l’Oignon.” Bull, du Mus. 
d’hist. Nat., 1902. 
Chauveaud. Sur la persistence de la structure alterne . . . . du 
Lamier blanc. Comptes rendus, 1904. 
6 T. G. Hill. “ On the Seedling Structure of certain Piperales.” 
Annals of Botany, 1906. 
8 In a few instances the bundles are grouped in diagonal pairs, 
composed of one central and one lateral, and the poles of the 
tetrarch root do not lie in the cotyledonary and inter-cotyle¬ 
donary planes, but are formed in the diagonal planes. ( Achras , 
Quercus, Fagus). Miss Winifred Smith first drew my attention 
to this arrangement in the Sapotacea 
