CHAPTER I.

INTRODUCTORY REMARKS.

Various means which favour or determine the cross-fertilisation of plants. 
Benefits derived from cross-fertilisation. 
Self-fertilisation favourable to the propagation of the species. 
Brief history of the subject. 
Object of the experiments, and the manner in which they were tried. 
Statistical value of the measurements. 
The experiments carried on during several successive generations. 
Nature of the relationship of the plants in the later generations. 
Uniformity of the conditions to which the plants were subjected. 
Some apparent and some real causes of error. 
Amount of pollen employed. 
Arrangement of the work. 
Importance of the conclusions.

There is weighty and abundant evidence that the flowers of most kinds of plants are constructed so as to be occasionally or habitually cross-fertilised by pollen from another flower, produced either by the same plant, or generally, as we shall hereafter see reason to believe, by a distinct plant.  Cross-fertilisation is sometimes ensured by the sexes being separated, and in a large number of cases by the pollen and stigma of the same flower being matured at different times.  Such plants are called dichogamous, and have been divided into two sub-classes:  proterandrous species, in which the pollen is mature before the stigma, and proterogynous species, in which the reverse occurs; this latter form of dichogamy not being nearly so common as the other.  Cross-fertilisation is also ensured, in many cases, by mechanical contrivances of wonderful beauty, preventing the impregnation of the flowers by their own pollen.  There is a small class of plants, which I have called dimorphic and trimorphic, but to which Hildebrand has given the more appropriate name of heterostyled; this class consists of plants presenting two or three distinct forms, adapted for reciprocal fertilisation, so that, like plants with separate sexes, they can hardly fail to be intercrossed in each generation.  The male and female organs of some flowers are irritable, and the insects which touch them get dusted with pollen, which is thus transported to other flowers.  Again, there is a class, in which the ovules absolutely refuse to be fertilised by pollen from the same plant, but can be fertilised by pollen from any other individual of the same species.  There are also very many species which are partially sterile with their own pollen.  Lastly, there is a large class in which the flowers present no apparent obstacle of any kind to self-fertilisation, nevertheless these plants are frequently intercrossed, owing to the prepotency of pollen from another individual or variety over the plant’s own pollen.