EXPERIMENTS IN PLANT HYBRIDISATION. 



7 



of trees, and strings stretched between. For each trial a number of pot 

 plants were placed during the blooming period in a greenhouse, to serve as 

 control plants with respect to the main trial in the open as regards 

 possible disturbance by insects. Among the insects* which visit Peas the 

 beetle Briicliiis pisi might be detrimental to the trials should they 

 appear in numbers. The female of this species is known to lay the 

 eggs in the flower, and in so doing opens the keel ; upon the tarsi of one 

 specimen, which was caught in a flower, some pollen grains could 

 clearly be seen under a lens. Mention must also be made of a circumstance 

 which possibly might lead to the introduction of foreign pollen. It occurs, 

 for instance, in some rare cases that certain parts of an otherwise quite 

 normally developed flower wither, which results in a partial exposure of 

 the fertilising organs. A defective development of the keel has also been 

 observed, owing to which the stigma and anthers remained partially 

 uncovered. It also sometimes happens that the pollen does not reach full 

 perfection. In this event there occurs a gradual lengthening of the stigma 

 during the blooming period, until the stigmatic tip protrudes at the point 

 of the keel. This remarkable appearance has also been observed in 

 hybrids of PJiaseoliis and Lathyrns. 



The risk of false impregnation by foreign pollen is, however, a very 

 slight one with Pisum, and is quite incapable of disturbing the general 

 result. Among more than 10,000 plants w^hich were carefully examined 

 there were only a very few cases where an indubitable false impregnation 

 had occurred. Since in the greenhouse such a case was never remarked, 

 it may well be supposed that Bruchus jnsi, and possibly also the described 

 abnormalities in the floral structure, were to blame. 



The Forms of the HvBRiDs.t 



Experiments which in previous years were made with decorative 

 plants have already afibrded evidence that the hybrids, as a rule, are 

 not exactly intermediate between the parental species. ^Vith some of 

 the more striking characters, those, for instance, which relate to the form 

 and size of the leaves, the pubescence of the several parts, &c., the inter- 

 mediate, indeed, was nearly always to be seen ; in other cases, however, 

 one of the two parental characters w^as so preponderant that it was difficult, 

 or quite impossible, to detect the other in the hybrid. 



This is precisely the case with Pea hybrids. In the case of each of 

 the seven crosses the hybrid character resembles that of one of the parental 

 forms so closely that the other either escapes observation completely 

 or cannot be detected with certainty. This circumstance is of great 

 importance in the determination and classification of the forms under 

 which the offspring of the hybrids appear. Henceforth in this paper 

 those characters which are transmitted entirely, or almost unchanged 

 in the hybridisation, and therefore in themselves represent the hybrid 

 characters, are termed the dominant, and those which become latent in 

 the process recessive. The expression "recessive" has been chosen 



* [It is somewhat surprising that no mention is made of Thrips, which swarm in 

 Pea flowers.] 



t [Mendel throughout speaks of his cross-bred Peas as " hybrids," a iexm which 

 many restrict to the offspring of two distinct species. He, as he explains, held 

 this to be only a question of degree. — W. B.] 



