192 



NA TURE 



[7tdy^, 1874 



the Azores, and Madeira, and in the plains and low 

 valleys of Chili, Monte Video, Tasmania, and New 

 Zealand. Their characteristic families are the Laurinea?, 

 Juglandere, Ebenacea:, Myricacre, Magnoliacea:,Aceracere, 

 Hippocastanea;,Campanulacece, CistiaceXjPhiladelphinias, 

 Hypericacea;, mixed however with a large number of 

 Leguminosa, Composit;e, Cupuliferje, Labiatie, &c. 



4. The fourth group is of plants of temperate climates 

 having annual means of 14^ to o" C, and these are named 

 Microtherms. In Europe they occupy plains from the 

 Cevennes and Alps to the North Cape, in Asia from the 

 Caucasus or Himalaya, to 65°, in Ainerica from 38' or 

 40°, to 60° or 65'\ They are also met with in Kergiielen, 

 Campbell, and the Malonine Islands, and the mountains 

 of New Zealand. No characteristic families are enume- 

 rated, as it is the absence of forms that are usually Meso- 

 therms and above all of Megatherms or Xerophiles, which 

 distinguishes this group. 



5. The fifdi group is of plants living in arctic or antarctic 

 regions, or high on mountains intemperate regions. They 

 need but little heat, and hence are called Hekistotherms. 

 One of their important characteristics is that they can 

 endure the absence of light during the time they are 

 covered with snow. Though no family belongs entirely 

 to this group, Mosses, Lichens, Grasses, Crucifers, 

 Saxifrages, Roses, and Composites bear a large propor- 

 tion to the whole. .Some species of Betula, Salix, 

 Empetrum, Vaccinium, and certain Conifers also are 

 Hekistotherm. 



6. The sixth group includes exceptional plants ; those 

 requiring a mean annual temperature of more than 30° C, 

 for which the name Mcgistothcrm is proposed. 



After the description of his proposed groups, M. de 

 Candolle at once faces an objection he sees is sure to be 

 raised, and that is the difficulty of classing a species 

 under any one particular group. His reply is that it is 

 always possible to do so if due attention is paid to the 

 conditions under which it lives, both by studying the 

 climatal conditions of its native country, and by experi- 

 mental culture. Fossil plants, he admits, can only be 

 classed by analogy ; but he very justly adds that in deter- 

 mining their botanic affinities in like manner there is gene- 

 rally notliing but analogy to rely on, flowers and fruits being 

 wanting. In answer to the possible objection that there 

 are transitions from one group to another, and that the 

 limits are arbitrary, he is content to reply that though a 

 classification based on botanical characters may be more 

 precise, the limits of geographical groups and of geolo- 

 gical periods are equally wanting in exactness. 



The fact that his physiological groups in no way 

 coincide with established botanical or geographical groups 

 is worth notice. All families that are at all numerous 

 in species are represented in more than one of these 

 physiological groups, and sometimes in them all. To 

 give only one instance, the Primulaceiu live in almost 

 all cold and temperate regions, and yet the Myrsi- 

 neaccK, which are their woody representatives, ate 

 found in the tropics. Even in genera which have not 

 many varieties of form, the same is the case. The 

 Cassias, for example, are mostly Megatherms or Meso- 

 therms, yet Cassia marylandica flourishes at Geneva, 

 where the winter minimum is sometimes 25" C. Some 

 willows flourish far north, yet Salix linmholliaiia 

 is met with in the district of the Amazon, and Salix 

 safsaf grows in Egypt. 



Is there any connection between the physiological 

 properties of plants and the form of their organs of 

 vegetation ? M. de Candolle thinks not. For example : 

 there is no recognisable difference between the forms and 

 tissues of ferns which we have to preserve in hot-houses 

 and those which will grow in the open air. There are 

 many facts such as these which seem to show that there 

 is no direct relation of cause and effect between the form 

 and those physiological qualities of plants which have 



reference to climatal conditions. There is rather a depen- 

 dence on some common cause which has influenced both 

 sets of phenomena, which M.de Candolle refers to heredity. 

 A species has a particular form because its ancestors had 

 a form more or less the same. It has certain physiological 

 qualities with reference to climate because the exterior 

 conditions which have been imposed on it through innu- 

 merable ages have prevented other qualities from being 

 developed and have secured the heredity of those which 

 have enabled it to live. This, he considers, is the key to 

 the explanation why a flora of any particular climate does 

 not present in the totality of its species any distinctive 

 peculiarities. Arctico-Alpine plants are of different 

 families, and it is impossible to point to any development 

 of an organ which cannot also be met with in tropical 

 plants. The ascendants of Arctico-Alpine plants have 

 lived together, and only certain of them have lived to- 

 gether through changes of temperature. Physiological 

 qualities may be changed in length of time when exterior 

 conditions have not changed in such a way as to cause a 

 species to perish. M.de Candolle lays great stresson thefact 

 we learn from the experience of horticulturists, that it is 

 much more rare to obtain any change in the power of a 

 plant to endure modifications of climate than it is to ob- 

 tain change of form. A period of greater length than the 

 historic period of Europe seems to be needed for a modi- 

 fication of physiological conditions ; witness the fact that 

 for some 3,000 years the date has been grown in Greece 

 and Italy without any success in getting the fruit to ripen. 

 The fact that physiological conditions are so much more 

 permanent than form is to M. de Candolle a strong argu- 

 ment in favour of his physiological groups. The impos- 

 sibility of making geographical groups perfectly true, to- 

 gether with the fact that the climates of each region have 

 changed from one period to another, is also claimed as 

 additional argument in favour. 



For the purpose of showing that these groups make the 

 facts of geographical botany, both of geological and pre- 

 sent times, more precise and more easy of discussion 

 as regards general laws, their distribution in Europe since 

 the commencement of the Tertiary period is taken as an 

 illustration. The works of Gceppert, Hecr, Unger, Garo- 

 vaglio, Ch. T. Gaudin, .Saporta, lic, have supplied M. do 

 Candolle with his data, and on comparing the fossil floras 

 with recent forms he has had no difficulty in classifying 

 them according to his groups. He, of course, goes on 

 the hypothesis that like fornrs have sprung from like an- 

 tecedents possessing like hereditary physiological pro- 

 perties. As an illustration that any uncertainty there 

 may be is within limits, he points out that though a fossil 

 Ficus might be taken for a Megathenn or Mesotherm, it 

 could never be mistaken for a Microtherm or Hekisto- 

 therm, since we do not now know any Ficus capable of 

 resisting such cold. A fossil Betula may have been Mi- 

 crotherm or Hekistotherm, but not Megatherm. 



Acting on these hypotheses he has reduced his results 

 to tabular form, prefacing the remark that his great diffi- 

 culty has been to class the dift'erent fossil floras according 

 to geological periods that could be relied on; stratifica- 

 tion and not pateontology being the only safe basis of 

 relative age grouping. 



Different climates prevailed in different parts of Europe 

 during the Tertiary period as well as now, and he urges it 

 must be recollected that when two fossil floras (faunas 

 equally so) which are much alike are met with in widely 

 separated latitudes, they cannot have been contempo- 

 raneous. In the same latitude, too, difference of eleva- 

 tion will have had a similar effect to difference of latitude. 

 Floras of quite different facies may therefore have been 

 contemporaneous. 



In transcribing the following table and explanations we 

 have given only the name of the author who has described 

 the floras. M . de Candolle gives exact references to the 

 works where the descriptions may be found. 



