Fertilisation 



I2S 



the calyx to the ovary, or the epipetalous or gynandrous arrange- 

 ment of stamens, would be due to adhesion. 



It will often give an idea of how the parts of a flower are 

 arranged with regard to cohesion and adhesion to make a floral 

 diagram of it. Figs. 238, 239, and 240 are examples of how 

 this should be done. In fig. 238 we have in a Rose a poly- 

 sepalous polypetalous apocarpous regular flower, whilst in the 

 Potato (fig. 239) we have a gamopetalous syncarpous flower. 

 The diagrams show the different way that the petals and carpels 

 are represented in the two cases. Fig. 240 shows an irregular 

 flower, as seen in the Sweet-pea. 



Fertilisation. — In order that the ovary and ovules may 

 properly perform their functions, it is necessary that they should 

 be fertilised by the pollen. The 

 pollen grains adhere to the stigma 

 (fig. 241, b\ and being nourished 

 by the viscid secretion present, a 

 portion of the intine protrudes 

 either through an opening already 

 existing in the extine, or forcing 

 its way through a part where the 

 outer walls are thinner. The pro- 

 longation thus formed is known 

 as a pollen-tube (fig. 242). These 

 tubes grow down the loose con- 

 ducting tissue of the style and enter 

 the ovary. 



The time necessary for this 

 varies from a few hours to several 

 weeks. In the Crocus it takes from 

 twenty-four to seventy-two hours ; 

 in the Arum over five days, and 

 in the Orchids even some months. 

 Just previous to this, or about 

 the same time, a change has been 

 going on in the embryo-sac of 

 the ovule. Usually one or two cells are produced within the 

 embryo-sac at the end away from the micropyle. These are 



Fig. 241. — Longitudinal section 

 through the uniovular ovary of 

 Polygonum Convolvulus at the 

 time of flowering : a, stigma ; b, 

 pollen grains ; c, poUen-tube ; d, 

 wall of the ovary gjnj the erect 

 orthotropous ovule ; se^ its embryo- 

 sac : cht chalaza ; two pollen-tubes 

 have penetrated through the con- 

 ducting tissue of the style, one of 

 which has entered the micropyle of 

 the ovule, the other not. 



