DICOTYLEDONS. 649 



the daughter-cells or endosperm takes place only in the lipper 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 Cuscutd) 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 Tropaiolum 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 exlernal differentiation of parts {e.g. in 

 Monotropa^ Pyrola, Orobanche, Balanophoracese, and Raflflesiaceae). 



With reference to the Histology \ I will confine my remarks here to a description 

 of the behaviour of the fibro-vascular 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 ofl' 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. fiir wiss. Bot. vol. I. p. 233 et seq. ; and for the girdle-shaped combinations 

 of vascular bundles, Abh. der Berl. Akad. 1857, 8. — Nageli, Beitrage zur wiss. Bot. Leipzig, Heft I, 

 1858 ; and Dickenwachslhum und Anordnung der Gefassstrange bei den Sapindaceen, Munchen 1864. 

 — Sanio, Bot. Zeitg. 1864, p. 193 et seq., and 1865, p 165 et seq. — Eichler, Denkschrift der kon. bayer. 

 Bot. Gesells. vol. V. Heft I. p. 20, Regensburg 1864. — [De Bary, Vergleichende Anatomic der 

 Vegetationsorgane der Phanerogamen und Fame, 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. 



