49C) 



NATURE 



[December i6, 1920 



not even space-time coincidences, and not even 

 the dx's, dy's, dz's, and dt's (for these can have 

 significance only when they are referred to other 

 dx's, etc.), but the relations between these unsub- 

 stantial ghosts of reality and other similar ghosts. 

 And are not these relations most obviously the 

 products of the mind? If so, is not our 

 mechanism at the same time also vitalism? 



Plainly, then, there is occasion for strong and 

 resolute thinking in biology, as well as in physics, 

 and until that has been done there ought to be an 

 end to these back-number controversies. One fails 

 to find this in Prof. Thomson's book, but, none 

 the less, it is a book that most certainly ought to 

 have been written. It takes stock, so to speak, 

 of the situation of speculative biology at the begin- 

 ning of a new phase in science, and it does so in 

 a manner that is candid, comprehensive, and most 

 attractive. Even to have compiled the biblio- 

 graphy, for which a host of young biologists will 

 be thankful, is worthy of the gratitude of both 

 students and investigators. J. J. 



A Study of Weeds. 



Weeds of Farm Land. By Dr. Winifred E. 

 Brenchley. Pp. x-l-239. (London: Longmans, 

 Green, and Co., 1920.) Price 12s. 6d. net. 



IT is a healthy sign of the broad-minded, prac- 

 tical way in which agricultural research is 

 being conducted that this handsome book on 

 weeds should come from Rothamsted. It is neces- 

 sary to employ the utmost refinements of mathe- 

 matical and physical discussion in order to deter- 

 mine the water-retaining power of soil particles, 

 and to make recurring counts of the bacteria and 

 other organisms present in a gram of soil, if the 

 expert is to be furnished with the data he requires 

 in order to advise the farmer on the manuring and 

 cultivation of his soil. But the best of manures 

 will fail of effect if the land is not clean, and agri- 

 cultural investigators run the danger of perform- 

 ing harmonics on the academic string if they do 

 not constantly vitalise their thinking by watching 

 the farmer at work and learning from him" where 

 the real difficulties arise. 



Dr. Brenchley 's book deals with weeds, and 

 introduces us straightway to a big, and as yet 

 an unsolved, problem — that of competition among 

 living plants. As Rothamsted showed vears ago, 

 all the improvements in our agricultural plants, 

 that selection for immense vegetative capacitv 

 which makes them such excellent productive 

 machines, have not fitted them in the least to stand 

 competition. When the wheat crop on a part of 

 the Broadbalk field was left unharvested to sow 

 NO. 2668, VOL. 106] 



itself and recur to the wild life of struggle, it 

 persisted for only three seasons. By that time 

 the weeds had taken possession of the land, and 

 the wheat could hold its own no longer. A weed 

 is nothing more than a plant which can scratch 

 a living under the stress of competition with culti- 

 vated crops and in spite of the wholesale destruc- 

 tion wrought by tillage operations. 



Dr. Brenchley begins her book with a de- 

 scription of the devices by which weeds en- 

 sure their continuance and distribution. Some, 

 like the poppies, depend on the abundance 

 of their seed ; others, like the dandelion, havc 

 developed a plume or other device to spread 

 wide their seed ; others, again, like the creeping 

 thistle or couch grass, possess a creeping under- 

 ground stem which will yield a plant from every 

 fragment. This part of the philosophy of weeds 

 is easy ; we can see why particular weeds are 

 abundant, but why other equally well endowed 

 plants do not become weeds is less evident. As 

 in other domains of thought, teleology is a fine 

 weapon of argument until someone reverses it. 

 There are other factors in competition we do not 

 in the least understand. Why should certain 

 plants, chiefly European, harmless enough at 

 home, have such a deadly power of spreading and 

 becoming weeds in the worst sense when they 

 are let loose in Australia or America? The prickly 

 pear has rendered millions of acres unusable by 

 man or beast in Queensland ; every new country 

 shows some invader in impudent possession, 

 "having the time of its life." 



The factors in competition are as yet beyond 

 our summing; some slight reaction to soil or 

 climate may depress or improve the " constitution " 

 of the plant to a degree that is not apparent and 

 certainly not susceptible of measurement, yet that 

 slight change does determine whether the plant 

 can or cannot stand competition. Dr. Brenchley 

 supplies an example in point. Spurrey and sheep's 

 sorrel are perhaps the most useful of indicator 

 weeds — sure signs of soil acidity and lack of lime. 

 Spurrey may be said never to be seen on chalk 

 soils ; on the sour Bagshot sands it will over- 

 whelm the wheat. Yet these plants, when grown 

 in pots and plots free from competition, grow 

 better in limed or chalky soils than in their natural 

 medium. The same thing has been observed with 

 the calcifuge leguminous plants. 



The latter part of Dr. Brenchley 's book is occu- 

 pied with a discussion of the association of par- 

 ticular weeds with soils, especially arable soils, a 

 question of which she has made the first system- 

 atic study, and thereby upset a good many state- 

 ments that had been put out on insufficient ob- 



