ANGIOSPERMS. 389 



sac, and this gives support to the view, that we are dealing here with sporogenous 

 tissue which has become rudimentary in the course of development, and that 

 one usually of its cells and sometimes two may develope into macrospores directly, 

 and without previous division into four cells \ 



In ordinary cases then the embryo-sac so far displaces the tissue above it, that it 

 is at last covered by only a thin layer of it, or even comes into contact with the inner 

 surface of the inner integument, as in the Orchideae (Fig. 316 VII) \ in such cases the 

 tissue of the apex of the nucellus may remain intact, as in the Aroideae and some 

 others, or the apex of the embryo-sac may destroy it and project beyond it, either 

 extending into the micropyle, as in Crocus and the Labiatae, or even growing out 

 beyond it in the form of a long tube, as in Santalum. Sometimes also the embryo- 

 sac extends itself in its middle and lower parts and encroaches on the surrounding 

 tissue ; in many gamopetalous Dicotyledons it puts out vermiform prolongations, 

 which penetrate into and destroy the tissue of the integument, as in some Labiatae, 

 Rhinanthus and Laihraea. 



The egg-apparatus does not often show any variation from its normal number 

 of three cells. Santalum ^ as a rule has two oospheres, the origin of which is still 

 not certainly known. The synergidae in Satitalum and other plants (JValsonia, 

 Gladiolus, Crocus, Zea, Sorghum, Polygonum) have a long tubular prolongation with 

 distinct longitudinal striation \ The apex of the embryo-sac in Crocus, Gladiolus 

 and Santalum is pierced by the synergidae, as Schacht correctly stated and Strasburger 

 has confirmed, and their prolongations extend therefore beyond the embryo-sac. 

 But in the great majority of Monocotyledons and Dicotyledons the synergidae arc not 

 developed in so peculiar a manner and remain covered by the wall of the embryo-sac. 

 The cells of die egg-apparatus often conceal each other, and their number therefore 

 was often in former times incorrectly given. In all plants hitherto examined there are 

 as a rule two synergidae and one oosphere, but in Santahmi, as has been said, there 

 are two oospheres, an occurrence which has been exceptionally observed also in 

 Siningia, a genus of the Gesneraceae. Since the oosphere is more deeply placed in 

 the embryo-sac than the synergidae, the contents of the pollen-tube, when this has 

 applied itself to the apex of the embryo-sac, reach the synergidae first, or rather one 

 of them ; these however never experience any further development, but on the contrary 

 disappear, while the oosphere developes into the embryo, though it does not come 

 into contact with the pollen-tube *. The function therefore of the synergidae in the 

 process of fertilisation is only ancillary ; they serve to convey the fertilising substance 

 from the pollen-tube to the oosphere. 



' Godbel, Beitr. z. vergl. Entwicklungsgeschichte d. Spoiangien, II (Bot. Ztg. 1881}. 



^ LStrasburger in Ber. Deutsch. Bot. Ges. Ill, 1885, shows that in Santalum there is but one 

 oosphere ; the synergidae are large and constricted in the middle by processes of the wall of the 

 embryo-sac, and the portions below the constrictions have been taken for two oospheres, the oosphere 

 itself having been overlooked.] 



' Once known as the filiform apparatus. 



** That only one of the cells of the egg-apparatus (the germinal vesicle) can be regarded as the 

 oosphere, while the others, one or both, have only really the duty of conveying the fertilising 

 substance to the oosphere, has already been insisted on by Sachs in jiages 560 and 561 of his 4th 

 edition. [See also Strasburger on Safitalum in Ber. Deutsch. Bot. Ges. 1SS5, and his Neue 

 Unters. ii. d. Befruchtungsv. b. d. Phanerog.] 



