164 BOARD OF AGRICULTURE. 



It has occurred to me that it will be interesting to you to 

 examine these principles somewhat carefully, and I shall 

 therefore ask you to consider them at this time. So far as I 

 am aware, they are not given in detail in any English work, 

 although you will find an excellent digest in the text-book 

 of botany by Professor Sachs.* 



Professor Nageli's propositions will now be taken up in 

 their order, and each one will be slightly recast in order to 

 render the meaning clearer to those who have not made a 

 special study of botany. Many of the propositions will lose 

 their original form, but will retain their exact meaning. 



1. Only those plants zvhich are nearly related can cross. 

 Fertility, as a rule, does not go beyond the genus ; very often 

 it is restricted by the limits of the sub-genus, and sometimes 

 is confined to varieties of a single species. 



The different natural orders and genera are very diverse 

 in this respect.^ 



2. Plant-forms (varieties and species) hybridize with 

 greater difficulty^ and yield a smaller number of fertile seeds, 

 in proportion to the remoteness of their sexual affinity. This 

 sexual aiSnity is not identical with that relationship which 

 is recognized by external differences in form, color, and 

 habit, nor with that internal relationship which is based 

 upon chemical and physical constitution. Nevertheless, all 

 three affinities, as a rule, occur together, and are parallel to 

 one another. The action of the pollen upon the pistil is 

 more or less complete. Nageli enumerates the following 

 grades : — 



The first and lowest degree of action is that in which 

 merely the ovary, and perhaps the calyx, grows somewhat, 

 without appreciably affecting the ovules. A second grade is 

 marked by greater growth of the ovary, and by slight 

 enlargement of the ovules, which afterwards shrivel up. 

 The third degree has small, imperfect fruits, with empty 

 seeds ; a fourth has good normal fruits, with empty seeds ; a 

 fifth has normal fruits, with apparently perfect seeds which 

 have no germs ; a sixth, good fruits, with seeds which have 

 minute germs not capable of growth ; a seventh, good fruits 



1 A Text-Book of Botany. By Professor Sachs. Translated by Bennet & 

 Dyer. Oxford, 1875. 



2 For further explanation and details, see note A at the end of the lecture. 



