342 FOURTH GROUP.— SEED-PLANTS. 



the nuclei produced by the division of the nucleus of the oospore in Ephedra 

 continue free, while in Pinus they very soon become centres of union for cells 

 lying in the lower part of the oospore, but they may also occasionally remain 

 free (Strasburger). In Welwitschia the archegonium is reduced to a single cell 

 surrounded by a cell-wall. These rudimentary archegouia, from twenty to thirty 

 in number, grow out of the embryo-sac and penetrate into canal-like spaces in the 

 nucellus, where fertilisation is effected by the pollen-tubes which grow towards 

 them and lay themselves along them, while the wall of the archegonium swells 

 up at the point of contact. The oospore with its cell-wall elongates into a 

 tube, and a cell is separated off at its extremity and becomes a rudimentary 

 embryo ; when several cells have been formed in the rudiment the cells behind it 

 also grow into long tubes, so that the embryo when thrust downwards into the pro- 

 thallium terminates at its upper radicular extremity in unusually long embryonal 

 tubes ; but only one embryo is developed as the result of fertilisation in from two 

 to eight archegonia. An outgrowth, an organ of suction, is formed on the hypo- 

 cotyledonary portion of the embryo, which remains in contact with the endosperm 

 and supplies the young plant with nutriment from it. 



Appendix on the Histology of the Gymnosperms. 



From the abundant material which has been collected and critically examined in 

 De Bary's Comparative Anatomy, we can here only call attention to a few points which 

 are characteristic of the division. 



The vascular bundles are on the whole similar to those of the Dicotyledons ; there 

 is a system of common bundles, and their leaf-traces as they descend in the stem are 

 disposed in a circle, in which by means of interfascicular cambium a closed cambium- 

 ring is formed and gives rise to permanent growth in thickness ; the ascending limb 

 of each leaf-trace, which bends out into the leaf itself, assumes in the Cycadeae more 

 or less the character of a closed bundle, but retains the appearance at least of an open 

 bundle in the leaves of many Coniferae. No cauline bundles are formed in the stem of 

 the Coniferae or of Ephedra ; the leaf-trace-bundles descend through a number of inter- 

 nodes, and then lay themselves along older and deeper leaf-trace-bundles either on one 

 side only or on both sides by dividing into two limbs (Fig. 264). The leaves in the 

 Coniferae, with the exception of Gingko, receive only one bundle from the stem, 

 which usually divides in the leaf into two equal parts running alongside one another; if 

 the leaves are broader, the bundle that comes from the stem divides at the point where 

 it leaves the stem into several bundles which enter the leaf, as in Dammara and the 

 broad-leaved Araucarieae ; if the leaf forms a broad flat lamina, as in Gingko and 

 Dammara, the bundles ramify in it but without forming reticulations ; in Gingko they 

 form repeated bifurcations. These bundles seldom form prominent veins in the lamina 

 of the leaf in the Coniferae, but run through the middle of the tissue. In Ephedra 

 each leaf receives two, in Gnetwn four or five bundles {G. Thoa, De Bary, loc. cit.). 

 Many bundles enter the two huge leaves of Welwitschia, and their parallel ramifica- 

 tions run in the middle layer of tissue. Two bundles also enter the large pinnate 

 leaves of the Cycadeae, and these bend till they are almost horizontal in the cortical 

 tissue of the stem and divide in the leaf-stalk, when it is thick, into a number of strong 

 bundles elegantly arranged as seen in a transverse section ; in Cycas revoltita for 

 example they have the form of an inverted i2 ; they run parallel to one another in the 

 rhachis of the leaf and give oft" branches into the pinnae, where they either run parallel 

 in the middle layer of tissue, as in Dioon, or branch dichotomously, as in Encephalartos; 

 in Cycas they form a mid-rib which projects on the under surface. The course of the 



