26 FIELD AND WOODLAND PLANTS 



tubes will be seen to protrude, and these eventually grow to a 

 considerable length. 



In order that the ovules of a flower may develop into seeds, it 

 is necessary that they become impregnated by pollen from the 

 anthers of the same species, and this is brought about in the following 

 manner : The pollen cells having been transferred by some means 

 to the mature stigma, they adhere to the surface of the latter, and, 

 deriving their nourishment from the secretion of the stigmatic 

 cells, as above described, proceed to throw out their tubes. These 

 tubes force their w^ay between the cells of the stigma and style, 

 and enter the ovary. Each tube then finds its way to one of the 

 ovules, which it enters by means of a minute opening in its double 

 coat called the micropyle, penetrates the embryo-sac, and reaches 

 the ovum or egg-cell. The ovule is now impregnated or fertilised, 

 and the result is that the ovum divides and subdivides into more 

 and more cells till at last an embryo plant is built up. The ovule 

 has thus become a seed, and its further development into a mature 

 plant depends on its being transferred to a suitable soil, with proper 

 conditions as to heat and moisture. 



If the flower concerned is a perfect one, and the ovules are 

 impregnated by pollen from its own anthers, it is said to be self- 

 fertilised ; but if the pollen cells that fertilise the ovules have 

 been transferred from a distinct flower, it is said to be cross- 

 fertilised. 



Now, it has been observed that although self-fertilisation will 

 give rise to satisfactory results in some instances, producing seeds 

 which develop into strong offspring, cross-fertilisation will, as a 

 rule, produce better seeds. In fact, self-fertilisation is not at all 

 common among flowers, and the pollen has frequently no efi'ect 

 unless it has been transferred from another flower. In a few cases 

 it has been found that the pollen even acts as a poison when it is 

 deposited on the stigma of the same flower, causing it to shrivel 

 up and die. In many instances the structure and growth of the 

 flower is such that self-pollination is absolutely impossible ; and 

 wdiere it is possible the seedlings resulting from the process are 

 often verj' weak. 



It has already been hinted that the wonderful variety of form 

 and colour exhibited by flowers has some connexion with this 

 important matter of the transfer of pollen, and the reader who is 

 really interested in the investigation of the significance of this great 

 diversity will find it a most charming study to search into the 



