HYBRIDIZATION. 641 



spores of Ozothallia vulyaris, and vice versa. Neither could the spores of 

 Himanthalia lorea be fertilized by Ozothallia vulgaris or Fucus serratus, 

 nor the spores of Fucus serratus by the spermatozoids of F. vesiculosus. 

 But the spores of F. vesiculosus impregnated by the spermatozoids of F. 

 serratus became fertile and germinated ; which fact is the more interesting 

 since F. vesiculosus is a variable plant in its natural state, while the others 

 named exhibit comparative fixity. 



Cross-breeding in Phanerogamia. The existence of hybrids in Phane- 

 rogams, produced by impregnating the ovule of one plant with the pollen 

 of another, is a well-ascertained fact ; and indeed hybrids are produced 

 at will, within certain limits, by gardeners. 



It is necessary to distinguish here between true hybrids, or mules, result- 

 ing from the crossing of distinct species, and simple cross-breeds (or metis) 

 commonly included under the name of hybrids by gardeners, and resulting 

 from the crossing of varieties (p. 159) of the same species. It is from the 

 last operation that the great majority of the "hybrids" produced in 

 favourite florists' flowers, such as Pelargonium, Fuchsia, &c., are derived 

 this cross-fertilization usually presenting little difficulty, and commonly 

 occurring naturally where large quantities of varieties are grown together. 



The ready cross-fertilization of varieties spontaneously, places great 

 difficulty in the way of growing the varieties of cultivated vegetables for 

 seed. The different varieties of the Cabbage, Turnip, Pea, &c. are diffi- 

 cult to preserve pure as seeding-plants in large gardens or seed-growing 

 establishments, from the fact of insects and the wind carrying the pollen 

 from plant to plant. 



True hybrids, as a rule, subject to some remarkable exceptions, such 

 as Philayeria, Elisena, and Ismene, &c., can only be produced between 

 plants belonging to the same genus. When they are more diverse than 

 this, they will usually not cross ; and even within the limits of genera, 

 species wih 1 not always breed together. 



Generic difference in Flowering plants usually involves difference in the 

 structure of the reproductive organs, the size of the pollen-tubes, &c. ; we 

 are therefore not surprised at the above statement ; numerous instances, 

 however, occur of the refusal of nearly allied species to cross, where we 

 cannot detect any structural differences between them. It has also been 

 shown that in heterogonous flowers (p. 636) the different forms of the 

 same species are more or less sterile when intercrossed. 



The tendency to cross-breeding is less common than is frequently sup- 

 posed. Gaertner, the greatest experimental authority on this point, 

 states that in 10,000 sets of experiments, carried on during many years, 

 he only obtained 259 true hybrids. It is found impossible, for example, 

 to cross the Gooseberry and the Currant (two species of Ribes), the Apple 

 and Pear (two species of Pyrm), the Blackberry and Raspberry (allied 

 species of Rubus), &c. 



Besides this peculiar indisposition to hybridize, there exists an obstacle 

 in nature, in the greater facility with which an ovule receives the influ- 

 ence of its own pollen ; Gaertner describes this phenomenon under the 

 name of elective affinity, stating that when the natural pollen and that of 

 another species are placed upon a stigma, the foreign pollen remains 

 inert : and even when the natural pollen is applied a little time subse- 

 quently to the foreign pollen, it acquires the supremacy, and the embryos 

 prove true, and never hybrids. 



2T 



