PHENOMENA OF HYBRIDISATION IN THE GENUS MENTHA. 179 
werthiana F. Schultz, Malinvaldi G. Camus), but generally have little 
fixity.* 
Hitherto in France no authentic case has been known of spontaneous 
crossing between WM. arvensis and sylvestris, but examples of it have 
been observed in other countries. 
A powerful vegetative system (rhizomes, stolons and suckers, pseudo- 
rhizze), supplementing the sexual organs which are habitually imperfect, 
favours the propagation of hybrid Menthe, most often at the expense of 
their parents, which they eliminate, in some cases even completely, from 
the ground, which they occupy. A succession of individuals, continually 
derived from the same rhizomes for several years, may bring about the 
illusion that the form has the fixity of a species. 
By reason of the almost infinite variety of forms resulting from the 
polymorphism of Mentha and from the mutability of their hybrids, the 
nomenclature of the genus has grown to hundreds of names, of which 
many are only applicable to vanished types. 
I will briefly enumerate the principal signs of hybridisation : 
Inflorescence-—The characteristics drawn from the manner of arrange- 
ment of the inflorescence, on which is founded the Linnean division into 
spicata, capitata, verticillata,t are invariable in the true Menthe; all 
mixed inflorescences, that is to say those which exhibit on the same 
individual plant a combination or mixture of the above forms, are a 
certain mark of hybridisation, and such mixture including three forms 
of inflorescence is frequently seen in five species. In the groups 
aquatico-rotundijolia and aquatico-sylvestris, the inflorescence is almost 
always spicate at the top of the principal stalk, and often a globular head 
on the secondary axes ; these also terminate in a capitate form in many 
varieties of the sativa group, while the primary axis is surmounted by a 
cluster of small leaves, &c. 
Corolla.—The interior of this organ is glabrous in the legitimate 
forms of the three spicat@, and more or less pilose in aquatica and 
arvensis.t All departure from this rule is an evidence of hybridisation. 
The glabrous or pilose state of the exterior has no significance in this 
respect, 
* The same, Nos. 66 (Wohlwerthiana), 68 (Muelleriana), 69 (arvensi-rotundifolia 
Wirtg.), 70 (Malinvaldi G. Camus, = arvensis var. micrantha F. Sch.), 71 and 72 
(mollis and Scordiastrum F. Schultz, &c.). 
+ The terms spicate and verticillate are expressive here of the appearance. In 
fact we have to do with false spikes (spicastrwm) and with false whorls (verticil- 
lastrum), and it would be more correct to write spicastre and verticillastre. 
{ The Prussian botanist Wirtgen was the first to remark about half a century 
ago that the tube of the corolla was always glabrous in the interior (Blwmen-Kronen- 
réhre innen kahl) in the legitimate Mentha spicata and more or less pilose 
(“* Bl. innen behaart’’) in the species belonging to the two other groups. I have 
confirmed the accuracy of this observation, which is often a great help towards the 
discrimination of hybrid forms. Nevertheless Wirtgen exaggerates the value of it in 
substituting the new characteristic which he had found in the manner of inflorescence 
in order to make it the basis of an artificial classification which manifestly violated 
the natural affinities. In his Flora der preussischen Rhein-provinz (Bonn, 1857) 
he formed two sub-genera: (A) Mentha properly so called with M. piperita, 
viridis, gentilis, rotundifolia, sylvestris, pubescens, and (B) Trichomentha with 
M. aquatica, rubra, and arvensis. These arbitrary and inconsistent classifications 
manifestly transgress the principle of the subordination of characteristics, and, in 
consequence of this error of judgment, Wirtgen reaps but a small benefit from his 
; happy discovery. ; 
M2 
