207 
the Embryo and Seedling in the Gramineae . 
them are improbable. Fusion between two distinct and adjacent members 
occurs very commonly in all parts of the plant; there is no morphological 
reason why the stalk of the cotyledon should not unite with the hypocotyl 
to form the mesocotyl. More or less complete fusion of two separate 
bundles within the cotyledon is frequent among Monocotyledons, and may 
be supposed to occur in the sucker and stalk of type X. When these two 
changes have been accomplished, the sucker of the cotyledon will appear 
sessile on the axis at the base of the mesocotyl, while its trace runs upwards 
to the first node. In Arena the inversion of this trace, and its double 
character, suggest its origin from the two bundles of the stalk in a- form 
such as X. 
A third modification is necessary to convert the sheath of type X into 
the coleoptile of the Arena type. The double scutellum trace, representing 
the two stalk bundles of X , divides at the node, and if each half-trace 
behaved as in X , it would run nearly to the top of the sheath, and turning 
sharply down again join the mesocotylar stele at the first node. The more 
acute the angle made by the half-trace on itself, the nearer would the 
ascending and descending segments lie to each other. If they approached 
so closely as finally to unite from first node to apex, the coleoptile bundles 
of Arena would be reproduced. That such fusion between two lengths of 
the same bundle may actually occur is shown in the sheath of Tigridia 
(Text-fig. 8, p. 168, and PI. X, Fig. 15). 
The two sheath-bundles of type X finally enter the stele of the hypo¬ 
cotyl from opposite sides, and presumably remain distinct within it. In 
Arena the descending segments of the coleoptile bundles unite in the stele 
of the mesocotyl—the fourth modification required to convert type X into 
the Arena type. But in the Zea type, best represented in this respect by 
Coix, no such alteration in structure is demanded. The coleoptile half¬ 
traces remain distinct in the stele ; separated from each other by the 
ascending scutellum trace, which in this type is included within it. 
We have implied throughout this discussion that the Arena type is 
the most primitive of the three described by Van Tieghem. Indeed, this 
conclusion follows from our conception of the mesocotyl. For if it repre¬ 
sents the hypocotyl of a remote ancestor united with the stalk of its 
cotyledon, the two types possessing a mesocotyl are nearer to that ancestor 
than the third which has none. And in the Arena type the ancestral stalk 
is represented by a separate trace, which in the Zea type is absorbed in the 
stele of the hypocotyl. Thus on our hypothesis the vascular skeleton of 
the Arena seedling represents the ancestral type X more completely than 
that of Zea or Sorghum. 
But though, on the whole, the seedling skeleton of Arena satira has 
more points which suggest the imaginary type X than that of any other 
species we have examined, yet certain isolated characters are better repre- 
