EXPERIMENTS IN PLANT HYBRIDISATION. 



5 



Their systematic classification is difficult and uncertain. If we adopt 

 the strictest definition of a species, according to which only those indi- 

 viduals belong to a species which under precisely the same circumstances 

 display precisely similar characters, no two of them could be imputed to 

 one species. According to the opinion of experts, however, the majority 

 belong to the species P'mtm sativum ; while the rest are regarded and 

 classed, some as sub-species of P. sativum, and some as independent 

 species, such as P. quadratuvi, P. saccharatiim, and P.umhcllatum. The 

 positions, however, which may be assigned to them in a classificatory 

 system are quite immaterial for the purposes of the experiments in 

 question. It has so far been found to be just as impossible to draw a 

 sharp line betw^een the hybrids of species and varieties as between species 

 and varieties themselves. 



Division and Arrangement of the Experiments. 



If two plants which differ constantly in one or several characters be 

 crossed, numerous experiments have demonstrated that the common 

 characters are transmitted unchanged to the hybrids and their progeny ; 

 but each pair of differentiating characters, on the other hand, unite in 

 the hybrid to form a new character, which in the progeny of the hybrid 

 is usually variable. The object of the trial was to observe these variations 

 in the case of each pair of differentiating characters, and to deduce the law 

 according to w^hich they appear in the successive generations. The trial 

 resolves itself therefore into just as many separate experiments as there 

 are constantly differentiating characters presented in the trial plants. 



The various forms of Peas selected for crossing showed differences 

 in the length and colour of the stem ; in the size and form of the leaves ; 

 in the position, colour, and size of the flowers ; in the length of the 

 flow^er stalk ; in the colour, form, and size of the pods ; in the form and size 

 of the seeds ; and in the colour of the seed-coats and the albumen [cotyledons]. 

 Some of the characters noted do not permit of a sharp and certain separa- 

 tion, since the difference is of a "more or less " nature, w'hich is often 

 difficult to define. Such characters could not be utilised for the separate 

 trials ; these could only be confined to characters which stand out clearly 

 and definitely in the plants. Lastly, the result must show whether they, 

 in their entirety, observe a regular relation in their hybrid unions, and 

 whether from these facts any conclusion can be come to regarding those 

 characters w^hich possess a subordinate significance in the type. 



The characters which were selected for the trials relate : 



1. To the difference in the form of the ripe seeds. These are either 

 round or roundish, the wrinkling, when such occurs on the surface, being 

 always only shallow ; or they are irregularly angular and deeply wrinkled 

 (P. quadratum). 



2. To the difference in the colour of the seed albumen (endosperm).* 

 The albumen of the ripe seeds is either pale yellow, bright yellow and 

 orange coloured, or it possesses a more or less intense green tint. This 

 difference of colour is easily seen in the seeds, as their coats are trans- 

 parent. 



* [Mendel uses the terms "albumen" and "endosperm" somewhat loosely to 

 denote the cotyledons, containing food-material, within the seed.— W. B.j 



