REPRODUCTION. 241 



B. FERTILE SEEDS. HYBRIDIZATION. APOGAMY. 



In regard to the production of fertile seeds, the following factors 

 are of prime importance. 



I. Only one pollen-tube is engaged in fertilizing an ovule. If 

 pollen-grains from different species of plants fall upon the same 

 stigma, only that one will be active in fertilization which has the 

 greatest sexual affinity (see II). According to recent investigations, 

 it is necessary that some of the nuclear substance of the pollen-tube 

 (male pronudeus) fuses with the nuclear substance of the egg- cell 

 (female pronudeus). The manner in which the pollen-tube readies 

 the egg-cell has already been described. 



II. We must distinguish between systematic and sexual rela- 

 tionship; they are not identical. The latter may be ascertained 

 by methods of crossing ; the former we judge by the similar or dis- 

 similar characteristics. Plant-forms which are widely separate 

 systematically are often closely related sexually, that is, they can 

 be crossed; as, for example, Lychnis diurna and Lychnis flos 

 cuculi ; while Pirus malus and Pirus communis (apple and pear) 

 show only slight sexual affinity or perhaps not any. Species of 

 Dianihus are readily crossed ; species of Silene with difficulty or not 

 at all; Rosacem, Salicacece with comparative ease; Papilionacece 

 with difficulty ; etc. It is further interesting to note that while 

 a may be fertilized by 5, b will not be fertilized by a (non-recipro- 

 cal or imperfectly reciprocal hybridization). Different varieties 

 cross very readily (variety -hybrids), different species less readily 

 (species-hybrids), different genera very rarely (genus-hybrids). The 

 following statement is generally applicable : Only such plant-forms 

 as show a close systematic relationship can be successfully crossed. 

 This does not preclude the possibility that the fertilization between 

 varieties may be more effective than fertilization between two in- 

 dividuals of the same variety. 



III. Fertility and other conditions of hybridization. The 

 sexually produced offspring of two plant-individuals which do not 

 belong to the same systematic unity, but to different varieties, 

 species, or genera, are called hybrids (bastards}. The greater the 

 difference in the systematic affinity of the parents of a hybrid the 

 greater the liability to sterility. Widely separate species do not 



