180 REPORT OF THE CONFERENCE ON GENETICS. 
Calyx.—This is always more or less hairy at the base in the true 
verticillate and capitate forms; a complete absence of hairs at this point, 
at least between the nerves (a very marked characteristic in M. viridis, 
and habitually coinciding with the glabrousness or glaucousness of the 
interior of the corolla), indicates a crossing between M. arvensis and 
viridis (M. gentilis and its varieties), or between aquatica and viridis 
(M. citrata Ehbrh., M. odorata Sole, M. adspersa Moench). 
Many forms of the group sativa (aquatico-arvensis) are only distin- 
guished by the character of the calyx (tubular-campanulate with lanceolate 
acuminate teeth, as in M. aquatica) from M. arvensis, of which the 
calyx is campanulate-urceolate with short triangular teeth. This last 
characteristic is not, moreover, pronounced except in fertile individuals. 
Leaves.—Only the characteristics of those on the principal stalk 
must be considered. Normally they are sub-sessile in the Spicate, and 
distinctly petiolate in M. aquatica and M. arvensis ; disturbance of these 
characteristics furnishes a presumption of hybridisation. When we see 
on a Mentha of the Verticillate section, apart from all pathological or 
teratological conditions, shortening of the leaves in the middle of the 
stalk in comparison with those above as well as those below, resulting in 
a club-shaped appearance (forma strangulans), we may infer the interven- 
tion of a Mentha of the section Spicate, ordinarily M. rotundifolia. 
We need not specify the signs of hybridisation of less value; those 
we have just defined will nearly always reveal the double origin of the 
varieties represented. Doubtless we must expect to meet with complex 
and embarrassing cases amongst the phenomena of hybridisation ; these 
are, however, relatively few. 
The characters of hybrids result generally from various combinations 
and mixtures of those of the parents, and it is rare for them to present 
a simple juxtaposition as in the type form of Mentha Ayassei Mlvd.,* 
which presents the globular inflorescence derived from M. aquatica with 
the unmodified leaves of WM. sylvestris. 
To sum up, from my personal observations, which are spread over 
more than forty years, the hybridisation of M. aquatica with M. arvensis 
(=M. sativa) is the rule everywhere, where these two species are 
spontaneously crossed; it is the same with the production of rotundi- 
folio-sylvestris ; on the contrary, the hybridisation of the other Mentha 
hybrids is exceptional—above all, the arvensi-sylvestris; the aquatico- 
rotundifolia and the arvensi-rotundifolia are less common, sporadic, and 
unstable. The aquatico-sylvestris and the arvensi-viridis, still more rare in 
France than the preceding, appear to be very widely distributed in certain 
countries of Central and Southern Europe. The aquwatico-viridis, rotundi- 
folio-viridis, and sylvestri-viridis are usually of horticultural origin. 
Finally, I have hitherto sought in vain for an authentic case of 
the crossing of M. Pulegiwm with an Humentha, and the lack of affinity, 
which this negative fact makes evident, authorises us in establishing 
the genus Pulegiwm,t and, d fortiori, in admitting the genus Preslia 
(Mentha cervina) and Menthella (Mentha Requieni). 
* See Menthe exsicc. pres. gallica, Nos. 89 and 40. 
t+ We have seen, in 1879, in the herbarium of the Paris Museum, a Mentha 
furnished by the Grenier collection and named by Reuter M. arvensi-Pulegium, 
which showed all the characteristics of a M. sativa. 
