KN()\VLi:i)Gi:. 



JlNE, 1912. 



BOTANY. 



By Proiessor F. Cavers, D.Sc, F.L.S. 



THI-: SOIL AND THK PLANT.— Under tlii.s title. Dr. 

 Kiissrll has recently ^Science Progress, No. 21, 191 1) discussod 

 various problems raised by the researches of the past few 

 years into the relations between plants and the soil. He 

 points ont that both the chemical and physical properties of 

 the soil .ifToct the growth and health of the plant; also that it 

 is most iTnprobabIc, as suf^gcsted by sonic recent workers, that 

 the chemical properties of the soil are relatively insif^nilicant 

 in deterniinint," fertility. A deficiency in any one factor limits 

 the effectiveness of the rest, and many of the injurious or 

 inhibiting factors are profoundly influenced by the presence 

 or absence of calcium carbonate in the soil. He criticises the 

 various theories brouijht forward by the members of the 

 United States Bureau of Soils, regarding the concentration of 

 the soil solution, its constancy of composition, the to.xic nature 

 of certain soil water for plant growth, and the part played by 

 fertilisers. As to toxic or poisonous substances, it cannot be 

 taken as proved that a substance toxic in water culture is 

 necessarily toxic in the soil itself, as the soil possesses 

 absorptive properties. Moreover, no evidence of the exist- 

 ence of poisonous plant-excretions in the soil has yet been 

 obtained, and the continuous growth of wheat on one field at 

 Rothamsted for nearly seventy years is cited as a proof to 

 the contrary. 



In this review, it is clearly shown how complex is the 

 relationship between the soil and the plant, and how important 

 a part is played by the biological factors, as well as the 

 chemical and physical constitution of the soil, in determining 

 the total eflect upon plant growth. A list of the recent 

 literature on the subject is appended to the paper. 



BRITISH ELMS. — The Elms which occur native or 

 naturalised in this country are often difficult to determine, 

 owing to variation in the size and hairiness of the leaves, the 

 presence or absence of suberosity, and the occurrence of 

 hybrids. Ur. Moss has contributed to The Gardeners' 

 Chronicle a series of articles (G. C, Nos. 3718-3720) on the 

 British Elms, in which most of this confusion has been 

 cleared up. The various species and hybrids are critically 

 described and discussed, and a new variety is indicated. 

 The author points out, in dealing with the question of hybrid 

 Elms, that the early interpretation of the occurrence of such 

 forms — that plants yielding mixed seedlings were not good 

 species — has had a good deal to do with the reduction by 

 many botanists of British Elms to two or even to one species. 

 This view is discredited by the recent results of experiments 

 which show that a " pure line " may yield seeds producing 

 mixed seedlings if pollinated either by another " pure line " or 

 by a hybrid ; hence it is now necessary, before it can be said 

 that a plant of unknown origin is a hybrid, to self-pollinate the 

 plants and use only the seeds obtained by this means. If 

 such seeds yield mixed seedlings, it may be regarded as 

 established that the plant which produced them is a hybrid. 

 Even then, however, there is no proof of the parentage of the 

 hybrid. To obtain such proof, it is necessary to produce the 

 hybrid in question by cross-pollinating known plants, and 

 apparently this has not yet been done in the case of Elms. 



The formation of an excess of corky tissue is to be regarded 

 as an abnormality which may occur in any of our Elms, except 

 the Wych Elm iUlnius glabra). Such names as " Ulmiis 

 snberosa " or " Ulniiis canipestris var. snbcrosa," if founded 

 on the presence of suberous bark alone, are to be rejected. The 

 occurrence of suberosity is common in some cases {e.g., the 

 Dutch Elm) and rare in others (e.g., the Huntingdon Elnil. 

 The suberosity is commonest on the young branches produced 

 from adventitious buds low down on the trunk and on suckers. 

 Its cause is a matter for investigation by the plant pathologist 

 rather than the systematist, but it is interesting to note that 

 all our Elms which produce suberous bark also have suckers. 



Another cause of confusion is the smoothness or roughness 

 of the upper surface of the leaves. In the Wych FClni and English 

 Elm all the leaves are rough above, but the remaining Elms 

 are usually described as having the leaves smooth above. 

 This, however, only applies to leaves produced in Spring on 

 voung branchlets of the main branches (called " normal leaves " 

 in the Key). It does not apply to leaves formed on suckers, 

 or on twigs produced from adventitious buds low down on the 

 trunk, or on coppiced or cropped shoots, or on seedlings, or 

 even on the new shoots produced in summer on the main 

 branches — all these leaves are invariably rough above. This, 

 the author believes, has never been pointed out before; and in 

 assessing the value of old descriptions it is sometimes necessary 

 to reject all references to the smoothness or hairiness of Elm 

 leaves. 



The size of Elm leaves has also led to confusion, through 

 not allowing for the \-ariability in each species, variety, or 

 hybrid. On every shoot, of course, the size of Ivim leaves 

 varies considerably : and in the Key the descriptions of the 

 leaves refer only to the terminal leaves of each branch, unless 

 otherwise stated. By allowing for some variation, it is 

 possible to identify any British Elm by its normal leaves 

 alone. 



As the present writer has frequently had specimens of Elms 

 sent to him for identification by readers of " Knowledge " 

 and other students of our British trees, it may be useful to 

 reproduce here the key given by Dr. Moss at the conclusion 

 of his articles on the British Elms. Permission to do this has 

 been kindly granted by Dr. Moss and by the Editor of The 

 Gardeners' Chronicle. In the Key. an asterisk ( i indicates 

 that the tree is not indigenous in the British Isles, a dagger ( ' I 

 that it is doubtfully indigenous. The Wych Elm (C. glabra) 

 is indigenous throughout Britain ; U. nitens and U. sativa 

 are indigenous in south-eastern England and in the Eastern 

 Midlands (U. sativa is also possibly indigenous in Hampshire 

 and Glamorganshire! ; the Dutch Elm seems to be indigenous 

 in Cambridgeshire and no doubt elsewhere. 



I. 



Tree without suckers and without suberous bark ; branches 

 usually more or less arched : crown of tree large ; stamens 

 usually five or six, rarely four or se\en ; samara large (about 

 one inch long), seed in the centre ; lamina; always rough 

 above and acuminate, of the terminal leaves large (about five 

 inches long and nearly three broad), almost sessile. — Wych 

 Ei.M (U. glabra Hudson). 



Tree with suckers; bark suberous or not; stamens usually 

 four; seed usuallv between centre of samara and the notch. 



— II. 



II. 

 Tree \ery tall at maturity ; bole long, straight ; lower 

 branches^ wide-spreading ; crown large ; samara small (about 

 half an inch long), suborbicular ; laminae always hairy or 

 rough above, of the terminal leaves rather large (about three 

 and a half inches long and two inches broad) and acute- 

 acuminate, of the remaining leaves of each branchlet shorter, 

 suborbicular ; petioles about one-third of a inch long, h.airy. 



— 'English Elm (U. campestris L.) 

 Normal leaves smooth or glabrous above. — III. 



III. 



Samarae and laminae of the terminal leaves as broad as or 

 nearly as broad as those of the Wych Elm ; position of seed 

 variable. — \\ (Hybrid Elms). 



Samarae and laminae of the terminal leaves much narrower 

 than in the Wych Elm ; seed between centre of samara and 

 the notch. — \'. 



IV. 



Bole usually short ; lower branches widespreading and 

 usually very long ; crown very large ; laminae acute, of the 

 terminal leaves about four inches long ; petioles nearly a half 

 inch long, hairy. — DUTCH El.M ((/. glabra X nitens — (a) X 

 U. hollandica). 



§ The lower branches of hedgerow trees are usually lopped; and thus the typical habit is destroyed. In some districts, e.g., ii 

 Brittany, the branches are lopped almost from foot to crown ; and then the precise determination of the tree is a matter of difficulty 



