DICOTYLEDONS. 
649 
the daughter-cells or endosperm takes place only in the upper part which also 
encloses the oospore. This mode of formation of the endosperm differs however 
from that which occurs in the plants mentioned above, in taking place in the upper 
half of the embryo-sac by free cell-formation. 
In the very large majority of true parasites (except Cuscuta) and saprophytes, 
the endosperm is formed by cell-division ; in Cuscuta however by free cell-formation. 
Hofmeister states that only slight indications of the formation of endosperm are 
to be found in Tropccolum and Trapa, 
The mode of formation of the Embryo of Dicotyledons, as it has now been 
elucidated by Hanstein's recent researches, has already been explained in the 
introduction to Angiosperms (see Fig. 403). It need now only be stated in 
addition that in parasites destitute of chlorophyll and in some saprophytes the 
seeds become ripe before the embryo has emerged from the condition of a 
roundish mass of tissue still without external differentiation of parts {e. g. in 
Monoiropa, Pyrola, Orobaitche, Balanophoracese, and Rafflesiaceoe). 
With reference to the Histology \ I will confine my remarks here to a description 
of the behaviour of the fibro-vasciilar bundles and of the mode in which the stem 
increases in thickness. 
With the exception of a few water-plants of simple structure, in which a purely 
cauline fibro-vascular cylinder runs through the stem and increases in length at its 
summit, the foliar bundles originating from it later (in Hippuris, Aldro'vanda, Cerato- 
phyllum, and to a certain extent also Trapa, according to Sanio), it is the general rule 
that ' common ' bundles are first formed, the ascending branches of which enter the 
stronger foliage-leaves generally in large numbers, and then pursue their course as isolated 
bundles in the leaf-stalk and mid-rib, giving off the secondary bundles which constitute 
the venation of the lamina '\ The branches which descend into the stem mostly run 
downwards through several internodes, become first interposed between the upper parts 
of the older bundles, and sometimes (Fig. 465) first split and then coalesce laterally 
with the older bundles lower down. Sometimes (as in Iberis) every bundle is twisted 
in the stem and in the same direction, so that the bundles which have coalesced 
sympodially, belonging to leaves of different heights on the stem, ascend spirally within 
the cortex. But most commonly they run parallel to the axis of the stem, until they 
anastomose with older bundles lower down. The bundles do not bend deeply into the 
inner tissue of the stem, but turn downwards and run parallel to one another at the 
same distance below the surface, so that they lie in one layer, which presents the 
appearance of a ring on transverse section separating the fundamental tissue into pith 
and primary cortex. The portions of the fundamental tissue which lie between the 
fibro-vascular bundles connect the pith with the primary cortex, and form the primary 
Medullary Rays. If there is no subsequent increase in thickness no further change takes 
1 Hanstein, Jahrb. für wiss. Bot. vol. I. p. 233 et seq.; and for the girdle-shaped combinations 
of vascular bundles, Abh. der Berl. Akad. 1857, 8. — Nageli, Beiträge zur wiss. Bot. Leipzig, Heft I, 
1858 ; and Dickenwachsthum und Anordnung der Gefässstränge bei den Sapindaceen, München 1864. 
■ — Sanio, Bot. Zeitg. 1864, p. 1936/5^9'., and 1865, p 165 et seq. — Eichler, Denkschrift der kön. bayer. 
Bot. Gesells. vol. V. Heft I, p. 20, Regensburg 1864. — [De Bary, Vergleichende Anatomie der 
Vegetationsorgane der Phanerogamen und Farne, 1877.] 
^ When several fibro-vascular bundles enter a leaf-stalk, they are generally widely separated 
by the fundamental tissue ; but sometimes, as in the Fig, the bundles are arranged in a circle on 
transverse section, and form a closed hollow cylinder which divides the fundamental tissue of the 
leaf-stalk into pith and cortex. Isolated fibro-vascular bundles also run into the pith of the leaf-stalk 
in the Fig, as occurs also in the stems of some Dicotyledons. 
