152 



plish fertilisation quicker than the pollen of plants belonging 

 to different species. The action of such foreign pollen being 

 thus excluded, hybridisation is prevented. In other cases 

 the plants resulting from hybridisation may be unable to 

 establish themselves in nature, owing to the severity of the 

 struggle for existence with other plants. Among the char- 

 acters which are frequently, possessed by hybrids and which 

 serve to distinguish them from their parents are an increased 

 tendency to variation, a more luxuriant and vigorous vege- 

 tative growth, and larger, more brilliantly coloured flowers, 

 which also often tend to become double. For this reason 

 alone hybrids are important in horticulture. The great 

 importance of hybridisation, however, depends chiefly on the 

 fact that it affords a means of combining in one plant the 

 valuable attributes of several, and a species yielding a valuable 

 commercial product which is liable to damage by frost, or to a 

 particular form of disease, may thus be rendered hardy by 

 crossing it with an allied, but hardy, species, the hybrid off- 

 spring, while yielding the valuable product of one parent, 

 possessing also the hardy attributes of the other. If such 

 a desirable hybrid is obtained it would of course be carefully 

 isolated and cultivated in such a way as to prevent, as far 

 as possible, intercrossing with other plants. At the same 

 time it must be remembered that the practical attainment 

 of such a result is attended with great difficulties, the greatest 

 obstacles to successful work in this direction being the 

 inconstancy of many hybrids and the comparative sterility 

 of others. For successful hybridisation the flowers of the 

 plant chosen as the mother (if hermaphrodite) should be 

 deprived of their stamens before the latter are mature, the 

 flowers then being covered with paper caps to prevent the 

 access of insects and possible fertilisation by the pollen of 

 other plants. With a clean camel-hair brush the pollen is 

 then taken from the plant selected as the father and gently 

 and lightly applied to the stigmas of the protected mother 

 flowers. The caps are then replaced on the latter. If the 

 cross has been successful the flower will quickly wither, and 

 if this does not occur the cross should be repeated. 



137 - If we examine a lar g e number 

 Variability, of adult plants all raised from the seed of a single 

 individual, we shall find a few exceptionally tall, and 

 a few exceptionally short, specimens, while the great 

 majority will be seen to be intermediate between these two 

 extremes, of moderate, or medium height, and differing very 



