The Science of Plant Growing 



male flowers occur on one individual, and only female on another, as in 

 the Willow (fig. 53), Poplar, Aucuba, and Ash. 



Pollination and Fertilization. When pollen is carried from the 

 stamens of one flower by insects, the wind, or other agency, and deposited on 

 a stigma of another flower, the process is called pollination. If the pollen 

 is placed on the stigma of the same flower, that would be self-pollination; 

 and if the flower accomplishes this itself, as it frequently does, that would 

 be automatic self-pollination. The 

 word fertilization is often loosely 

 used to imply the same act, but no 

 fertilization can really take place 

 till the pollen tube has reached the 

 germinal vesicle in the ovule and 

 formed a union with it. 



Sexual Reproduction. Ex- 

 cept in minor details this is ac- 

 complished much in the same way 

 in all vascular plants, which in- 

 clude Ferns, Selaginellas, and their 

 allies. The pollen grains of a 

 flowering plant are equivalent to 

 the microspores of a Selaginella; 

 and the anther in which they are 

 produced to the microsporangiurn 

 of Selaginella. The germinal 

 vesicle or egg cell is equivalent to 

 the megaspore, and the embryo 

 sac in which it is produced to the 

 megasporangium of a Selaginella. 

 The pollen is the male element and 

 the egg cell the female. A pollen 

 grain has two coats or skins, and 

 when it reaches the stigma the 

 inner coat soon after protrudes 

 and grows down the loose, con- 

 ducting tissue in the interior of the stigma and style, in the form of a 

 blind or closed tube (fig. 54), then passes into the ovary, and enters the 

 micropyle or opening of the ovule until it comes in contact with the egg 

 cell (fig. 56). The growing or elongating pollen tube contains two nuclei, 

 and the first and larger one fuses with the nucleus of the egg cell and 

 fertilization is complete. In other words, the male and female elements 

 have united to form one cell, which forthwith develops into an embryo 

 or new individual. If the pollen has been brought from another plant 

 of the same kind, the embryo will inherit the characters of both parents, 

 and, though it may not seem to differ much from either when it grows 



into a plant, it is usually more vigorous than an embryo which is the 

 VOL. i. 5 



Fig. 54. Flower of Cistus: Sepals and Petals removed. 

 The Stamens are hypogynous, and some of them have 

 their Anthers in contact with the Stigma. The Pollen 

 Tubes are shown passing down the Style and entering 

 the Ovules. 



