270 



BOTANY OF THE LIVING PLANT 



The protoplast of the perforated cell is not killed, and it may even 

 continue its movements for a time, and retain its turgescence. The 

 tube passes out between the walls of the subjacent conducting cells, 

 and continues its course in that way. The Mallow behaves similarly, 

 with the further feature that a number of tubes are formed from 

 each of the large grains ; but only one develops of large size, the 



rest serve to fix the grain on the 

 surface of the stigma, the large tube 

 conveying the essential contents 

 (Fig. 212, 5). It thus appears that 

 pollen-tubes behave upon the sur- 

 face of the stigma like the filaments 

 of parasitic Fungi, which similarly 

 either follow the surface of the in- 

 vaded tissue or grow between its 

 cells ; but sometimes they penetrate 

 the cells themselves. There is no 

 doubt that in its course the pollen- 

 tube also draws nourishment from 

 the tissue it traverses. 



Passing thus down the style and 

 into the cavity of the ovary, the 

 tube is often conducted mechani- 

 cally by directing hairs towards 

 the ovule, which in the common 

 inverted type has its micropyle 

 Fig. 214 tris. closc to the Wall of the ovary. 



A, embryo-sac of Helianthus annuus (after TVip lacf rkCirf- nf fVip rmirc/=. ic 

 Nawaschin). B, the male nuclei more highly "^ ^^ ^^^^ P^^^ ^^ ^^^ COUrSC IS 

 magnified. ^s = pollen-tube. sis.2=synergidae. believed tO be influenced bv the 



s/>i,s^.=maienuciei. oi;=egg-ceii. e;fe=centrai ueueveu Lu ue imiuencea uy tne 



fusion-nucleus of embryo-sac a = antipodal gyncrgids ! in SOmC CaSCS a drOD of 



cells, spi fertilises the egg ; spo fuses with the J i=> i f 



fusion-nucleus of the embry'o-sac. (From fluid dCHVed pCrhapS from them, is 



Strasburger.) ■ ' ir ir i 



exuded from the micropyle. What- 

 ever the influences may be, the tube enters the micropyle and 

 impinges closely on the apex of the nucellus ; where that tissue 

 has already been absorbed, it may advance directly upon the 

 embryo-sac, close to the egg-apparatus (Fig. 213, B). 



The passage of the pollen-tube direct to the micropyle is the usual, and 

 probably the primitive course. Fertilisation in that way is called porogamy. 

 But in a considerable number of plants it takes a course through the superficial 

 tissues of the ovule. Sometimes it passes through the funiculus to the chalazal 

 end of the embryo-sac, as in the Walnut and Casuarina \ this is called 



I 



