FLOWERS AND THEIR WORK. ' 217 



sugar solution on a cover-glass, dust some pollen in it, then invert 

 the oover over the ring. The drop will hang into a closed chamber, thus 

 preventing evaporation ; note the growth of the pollen-tubes. 



(c) The development of the embryo is easily followed in the Shep- 

 herd's Purse, a very common weed which flowers nearly all the year 

 round. Pick flowers from which the outer parts are falling or have 

 just fallen. Open the young fruit and remove the young seeds to a 

 slide, then oover them with a few drops of caustic potash solution. 

 Tease the seeds with needles, or press them under a cover-glass, to 

 isolate the embryos. Sketch the stages observed (Fig. 71). 



246, Sslf-Follination and Cross Pollination. In the 

 Broad Bjan, as in many other plants, it is found, on making 

 experiments in artificial pollination, that it is better if 

 the pollen cornos from a flower on another plant of 

 the same species (cross -pollination) than if it comes from 

 the anthers of the same flower or from those of another 

 flower on the same plant. The two latto cases ara very 

 similar in their results, and may be both included under 

 the term self-pollination. When cross-pollination occurs, 

 the resulting seeds are generally more numerous or heavier, 

 and give rise to stronger offspring (superior in height, 

 weight, fertility, etc?.) than is the case with self-pollination. 



In self-pollination there is a mixing of practically similar 

 characters, while in cross-pollination there is a mixing of 

 more or less dissimilar characters, especially when the plants 

 that produce tho male and female cells live far apart and 

 under different external conditions. Also, any useful varia- 

 tion ia likely to be transmitted and even strengthened whon 

 cross-pollination occurs. It is, therefore, an advantage to the 

 species to secure cross- pollination, and we find many adapta- 

 tions, some of them very remarkable, which favour cross- 

 pollination and hinder self-pollination. 



247. H3W Pjllen is Carried. Pollen- grains have no 

 power of spontaneous movement. When the anthers are 

 above the stigm a, self-pollination may occur (if the two are 

 mature at the sanvj tiinj) by pollen simply falling on the 

 stigma, but for cross-pollination the grains must be carried 

 to the stigma, of another flowjr by water, wind, or animals. 

 Water-pollination occurs only in water-plants, and only in 

 a few of these. In this country insects are probably the 



