314 BOTANY PART i 



which were orange-coloured. If an early blooming form were crossed with a 

 late bloomer, the hybrid would flower at a time intermediate between the two. 



A large number of spontaneous hybrids have been found which have arisen 

 naturally from plants with a special tendency to hybridisation. That such 

 natural hybrids do not oftener occur is due to the lack of an opportune time or 

 space for their development, and also to the fact that in the case of pollination 

 of flowers with different kinds of pollen, that of their own species seems as a 

 rule more effectual in effecting fertilisation ( 118 ). 



The investigations of DE VUIES, CORRKXS, TSCHERMAK, and others, have 

 greatly increased our knowledge of the laws governing the inheritance of different 

 characters of plants. No final conclusions are yet attained, however, since even 

 in regard to limited portions of the problem new features are still becoming 

 apparent ( 119 ). 



In the crossing of closely related forms (varieties or nearly related species), 

 Mendel's laws (called after their first discoverer, GREGOR JOH. MENDEL [1866] are 

 found to hold. These laws, the most generally applicable of which is that of the 

 segregration of characters, were independently rediscovered by DE VRIES and 

 CORRENS. An example will give the best idea of these laws. If a red-flowered 

 Mirabilis jalapa be crossed with a white-flowered individual one obtains a 

 generation of hybrids with uniformly rose-coloured flowers. If these are fertilised 

 from one another a second generation is obtained, but the individuals of this are 

 not uniformly coloured ; in addition to rose-coloured plants pure red-flowered 

 and white-flowered plants occur in the proportion per cent of 50 : 25 : 25 i.e. 

 in the ratio 2:1:1. When fertilised from one another the pure red-flowered 

 plants produce a red-flowered progeny and the white-flowered plants also breed 

 true ; they have returned to the pure parent forms. The 50% of rose-coloured 

 plants again splits in the next generation, and like the former generation yields 

 25% pure red, 25% pure white, and 50% rose-coloured plants. The proportion of 

 hybrid plants thus continually becomes lessened by the return to the red and 

 white types ; in the eighth generation only 0'75% of hybrids remain, and this 

 small remainder continues to split further on breeding. These results are 

 theoretically explained by assuming that the sexual cells of the rose-flowered 

 hybrids are not themselves of hybrid nature, but are already segregated into 

 pure red and pure white sexual elements. In the process of fertilisa- 

 O~O tion the union producing a hybrid, red x white (white x red, red x white) 

 y will occur twice as frequently as the union red x red or white x white 

 which give rise to pure forms. This is illustrated in the accompanying 



diagram in which the dark circles represent the red sexual elements. 

 The characters in which the parents differ do not, however, always blend so 

 that the hybrid exhibits an intermediate character. More usually the hybrids 

 completely resemble in this respect either the paternal or maternal parent, the 

 character of the one parent being dominant in the hybrid while the other 

 remains latent (Law of. dominant characters). This is the case for example in 

 hybrids between Urtica pilulifera with serrate leaves and U. Dodarti (Fig. 246). 

 The hybrids have all serrate leaves like U. piliiliftra, so that in the second 

 generation the proportion of serrate-leaved to entire-leaved individuals is per cent 

 75 :25 (3 : 1). Only 50% of the serrate-leaved individuals are, however, of hybrid 

 nature and continue to show a similar splitting of characters in the next genera- 

 tion ; 25% have become pure U. piluUfera. It is impossible to predict which 

 characters will prevail in any cross, and the question can only be settled by experi- 

 ment ; usually the phylogenetically younger character appears to be dominant. 



