762 



LECTURE XLII. 



of cellulose (the so-called Inline) of the pollen-grain which grows out and forms the 

 pollen-tubes. The places at which this will take place are, as already stated, indicated 

 beforehand, and sometimes, as in the Cucurbitaceae, circular excised pieces of the 

 e'xdne are set like lids into the apertures, and are simply pushed aside by the 

 emerging pollen-tubes, as in Fig. 443. The pollen-tubes have now to make their 

 way through the tissue of the stigma and style down into the cavity of the ovary ; 

 inside the latter they either enter directly into the micropyle of the ovule, which how- 

 ever occurs but seldom, or, as is more usual, they grow down along the walls 



of the ovary until they reach the 

 micropyle. To this end the paths 

 to be traversed are often indicated 

 beforehand by special relations of 

 organisation on the wall of the ovary. 

 The pollen-tubes of the Angiosperms 

 are for the most part very delicate, 

 their walls being relatively thick; 

 they may thus be easily passed over 

 in the tissue of the style, and it is 

 always one of the difficult tasks of 

 microscopy to discover their fertilis- 

 ing end penetrating into the micro- 

 pyle. Sometimes the distance which 

 the pollen-tubes have to traverse 

 from the stigma to the micropyle, 

 is very considerable — e.g. in the 

 Maize it is 20-40 cm. and in many 

 other long-styled flowers it is from 

 3-10 cm. 



The effect of fertilisation makes 

 itself evident chiefly in two points, 

 in the development of the oosphere 

 into the embryo, and in the in- 

 ception of the endosperm. 



By means of rapidly repeated bi- 

 partidons of the nucleus of the em- 

 bryo-sac, as Strasburger has shown, 

 there are produced in a short time 

 as a rule very numerous nucleated 

 masses which distribute themselves at regular distances from one another in the 

 protoplasmic lining of the embryo-sac. Around each of these nuclei a portion of the 

 protoplasm collects as around a centre of attraction. At the boundaries of these 

 portions of protoplasm arise thin cellulose walls (cf. Fig. 106, p. 106) and thus is 

 formed a layer of tissue which is at first applied to the wall of the embryo-sac, the ex- 

 ternal layer of endosperm ; as its cells grow in towards the interior of the embryo-sac, 

 and undergo transverse divisions parallel to its wall, the cavity of the embryo-sac is 

 gradually filled up with cell-tissue. If the embryo-sac is very narrow, or even tube-like 



Fig. 44^.—^ a pollen-ijram (in section) germinating on the stigmatic 

 lobes. The outer membrane (extine) has a number of circular openings 

 closed by the lids d ; the inner membrane of the pollen-grain (;) is thick- 

 ened beneath the lid— it swells, protrudes through the hole, and thus 

 pushes aside the lid. One of these swollen cushions of the intine is grow- 

 ing into the stigmatic tissue as tlie pollen-tube sp, B a piece of the pollen 

 membrane ; i intine, c extine, a" a lid. 



