Seedling Structure of certain Centrospermae. 189 
served that the base of the hypocotyl, especially in the younger plants, is 
distinctly swollen and bears on one side a prominent peg or foot ( p ). 
Charles and Francis Darwin 1 have drawn attention to these peculiarities 
in A. umbellata and A. arenaria , and have figured them in the former plant ; 
they compare the swollen hypocotyl to a corm, and point out that the peg 
functions in rupturing the coat of the fruit. 
Avebury 2 also has described the seedlings 
of Abronia , and remarks that in the seed 
one of the cotyledons is almost aborted and 
remains quite small for some time after 
germination. In A. umbellata it afterwards 
becomes the largest and grows like an ordinary 
leaf. He further remarks that the full-grown 
cotyledons of A . arenaria are very unequal 
in size. 
Abronia umbellata , Lam. In the seedling 
about to be described there was one large and 
one small cotyledon. The large seed-leaf 
(Diagram 4, c. 1) has one large central bundle 
with many smaller ones on either side. Within 
the blade the central bundle divides into two 
and shows a certain amount of rotation, so 
that some, often only one or two, protoxylem 
elements become isolated between the two 
half-bundles which move apart (see Diagram 6, 
Figs. 1-4). This is comparable to what obtains 
in some of the preceding plants, Amaranthus 
sylvestris and species of Gomphrena and 
Chenopodium for instance. But in no case in 
these plants is the isolation so well marked as 
in Abronia , Allionia , and other Nyctaginaceae. 
At the base of the blade the condition 
obtaining is shown in Diagram 4, Fig. 1 ; the 
laterals fuse on to the adjacent half-bundles, FlG ' 6 ' Natural 
hence the petiole has two strands, the half- 
bundles referred to above, partly facing one another and with a small group 
of protoxylem elements between them (Diagram 4, Fig. 2). At a lower 
level the two large strands divide, each into two, and clearly are in the 
first stage of rotation (Diagram 4, Fig. 3). In this condition they enter 
the axis. 
Turning now to the small cotyledon (c. 2), which is not nearly so well 
1 C. and F. Darwin : The Power of Movement in Plants. London, 1880. 
2 Lubbock : On Seedlings. London, 1892. 
