v.] MENTHA, MARJORAM, NEPETA. 153 



This geographical differentiation,, if it really exist, is 

 very interesting. 



H. Miiller attempts to explain the presence of 

 these small flowers by pointing out that where there 

 is any variation in the size of the flowers, the smaller 

 and less showy ones would be the last to be visited 

 by the insects. Under these circumstances, as such 

 flowers would be fertilised by the pollen derived 

 from previous visits, the stamens of such smaller 

 flowers would be useless, and would tend to become 

 rudimentary. Further observations are, however, 1 

 think, required before this explanation can be re- 

 garded as satisfactory. 



The Mint {Mentha arvensis) is also proteran- 

 drous, and, like the Thyme, possesses, in addition 

 to the hermaphrodite flowers, others which are 

 smaller and merely female. Some species of the 

 genus are dimorphous. The genus Mentha seems 

 to be in some respects a connecting link between 

 the typical Labiates, and the ordinary tubular 

 form. 



Origanum vulgar e (the Marjoram) also has plants 

 with large, proterandrous, bisexual flowers ; and 

 others with smaller female ones. In the secretion 

 and position of the honey it agrees with the Thyme ; 

 but while on the one hand it is less sweet, it is, on 

 the other, more conspicuous. These two differences 

 nearly counterbalance one another; the flowers are 

 consequently much visited by insects, and have also 

 lost the power of self-fertilisation. 



Nepeta glechoma (the Ground Ivy), like the three 

 preceding genera, is proterandrous, and has .'^mall 



