574 
Holden and Bexon.—On the Seedling 
under direct microscopic observation. Judged by these tests this substance 
is insoluble in alcohol, xylol, or benzene, but readily soluble in acetone, ether, 
and chloroform. It stains well with both Scharlach R and Sudan III, and 
gives an intense black with i per cent, osmic acid solution. 
Although its general staining and solubility characters are sufficiently 
indicative of its fatty nature it does not agree entirely with either the true 
fats or the lipoids described by Cramer ( 7 ), and it may be that more detailed 
investigations will show that it is a mixture of substances. 
o 
The Vascular System. 
(i) The Embryo and Young Seedling. 
The earliest stage studied in connexion with the development of the 
Fig. 6. Transverse section of a portion of the upper end of the hvpocotyl of a very young seed¬ 
ling before the radicle has penetrated the testa. Note the isolated protoxylem (J>x.) and the first- 
formed metaxylem elements (mx.). x 750. 
vascular system was that of the embryo at the stage when the fruit is shed 
from the tree in autumn. There is no differentiation of the xylem at this 
stage, but the position of the vascular strands is clearly indicated owing to 
the presence of large secretory sacs in the phloem areas. Embryos examined 
in the spring show the earliest stage of xylem differentiation, the protoxylem 
being well marked and the first metaxylem, though very immature, also 
being present (Fig. 6). Each seed-leaf is traversed by five main vascular 
strands, namely, the midrib, two laterals, and two marginals (Fig. 7). Towards 
the base of the leaf the midrib divides into three parts, these consisting of 
a central isolated protoxylem with a group of metaxylem and phloem 
forming a collateral bundle on either side (Fig. 8). These collateral bundles 
move outwards and unite with the lateral and marginal bundle so that at 
the cotyledonary node there are two isolated median protoxylem strands 
with four collateral bundles in the diagonal planes (Fig. 9). At some dis- 
