2/0 



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, B). 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 m- 

 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 

 close to the wall of the ovan,-. 



A, embrvo-sac of Hehanthus anniius (after xUr. loo*- ^ i- f i.\ 



Nawaschin/. B, the male nuclei more highly '^'^^ '^-St part Ot the COUrSC IS 



magnified. ;ls = pollen-tube. siS2=synergidae. KpUpvprl tn hp inflnpnr-Prl K,7 fJ,o 



s/>„sf2=maleouclei. oo = egg-cell. eft = central UCilCVea TO DC innucnced Dy the 



fusion-nucleus of embryo-sac. a = antipodal svnprcrids- in =;nmp PT^pc: q Hrr>r> of 



cells. s;>, fertilises the egg; s^., fuses mtb the ^ynerglUS , m SOme CaseS a Orop Ot 



s?raTburg'er-r °^ ""^ ^"''^'^°'^''''- """"^ Auid, derived perhaps from them, is 



exuded from tlie 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. 213,5). 



14 tt is. 



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 tlie embryo-sac, as in the Walnut and Casuarina ; this is called 



