76o 



LECTURE XLII. 



and of the Pseony. It is only in rare cases that, as in Bulomus, the ovules arise on 

 the whole of the inner surface of the carpel, and occasionally as may be seen in 

 Fig. 303, p. 466, an ovule appears to be the true end of the floral axis. 



In the main the ovule of the flowering-plants is not essentially different from 



-that of the Gymnosperms, although it usually differs in several external matters. 

 Fig. 441 ^ represents one of the commonest forms, an 'anatropous' ovule : on a 



-stalk-like funiculus KS is situated the nucellus of the ovule K invested by two integu- 

 ments i and i' and in such a way that the micropyle comes to lie next the base of 

 the stalk. There are, however, other forms also : straight or ' orthotropous ' ovules 

 as in Fig. 303, p. 466, for instance. Moreover there are not always two integuments, 

 but often (especially in many Dicotyledons) only one. The embryo-sac— and therein 

 lies an important difference in contrast to the Gymnosperms— often grows up as far 

 as the micropyle even before fertilisation (e mm£); and indeed cases are not rare 

 (Pedicularis and other Scrophularineae, and also Santalum) where the anterior end of 

 the embryo-sac grows out beyond the nucellus of the ovule, pushing its way into the 

 micropyle or even projecting out beyond it. However I shall here pay no further 

 attention to such peculiarities. 



A very important difference between the angiospermous flowering-plants and the 

 Gymnosperms comes out when the processes in the embryo-sac previous to fertilisation 

 are considered : the Angiosperms form no prothallium with archegonia, unless we 

 consider a sort of rudiment of them to exist in three cells which occur very frequently in 

 the hinder basal-end of the embryo-sac {u Fig. 442), and which bear the curious name 

 ' antipodal' cells— i. e. antipodal with respect to the oosphere. Starting from the con- 

 sideration of what occurs in the embryo-sac of the Gymnosperms, the proper repro- 

 ductive apparatus inside the embryo-sac appears to be a mere remnant, as it were, 

 which still retains only what is most essential and indispensable. Thus, in the anterior 

 end of the embryo-sac turned towards the micropyle there arise before fertilisation, 

 with rare exceptions, three naked cells close together. One of these, marked 2 in 

 Fig. 442, is the oosphere, from which the embryo is formed after fertilisation; the 

 two others, marked », which occupy the proper apex of the embryo-sac, were 

 designated by Strasburger the SynergidcB (Gehiilfinnen). Strasburger, to whom we 

 owe our more exact knowledge of the processes in the embryo-sac, assumes that it 

 is these synergidae which first take up the fertilising substance from the pollen-tube 

 and then pass it over to the oosphere. 



While these things are undergoing preparation in the embryo-sac, the pollen-sacs 

 of the anthers dehisce. By means of the wind, and much oftener by means of insects 

 or other animals seeking honey in the flowers, the pollen-grains are conveyed, to 

 the now moist and usually papillose surface of the stigma, where they remain attached, 

 and they put forth their pollen-tubes as a rule within a few hours (cf. i Fig. 443). 

 If there is only one ovule in the ovary, it is sufficient if one of the pollen-tubes grows 

 down through the style as far as the cavity of the ovary, and finally penetrates the 

 micropyle m, applying its end to the embryo-sac where it comes in direct contact 

 with the synergidae. Since, as is seen, each ovule requires a pollen-tube to fertilise 

 its oosphere, at least as many pollen-grains as there are ovules in the ovary must 

 reach the stigma and perfect the dev'elopement of their tubes. Not rarely however 

 an ovary contains hundreds and even thousands of ovules, and accordingly the dusting 



