Abstract : This is an interesting book for botanists, but it is of limited practical value to horticulturists, whether engaged in planting, research or advisory work. The present world production of rubber-like elastomers is about four million long tons, almost equally shared between synthetic and natural rubber; over 98 % of the latter is obtained from Hevea brasiliensis planted in Asia and Africa. However, this is riot reflected in the contents of the book: more than one-tenth of its 450 pages are devoted to useful indices and bibliography, about one-third concerns the botany of the various-plant sources, and only one-fifth is given to the production of rubber from them. Thus crop botany and production account for just over half the volume, but of this portion only a quarter is given to Hevea; further there is bias towards description of American practices in which Hevea husbandry, plant breeding and selection are dominated by the need to control South American-leaf blight, a disease happily absent from the non-American territories which produce most natural rubber.
The rest of the book is devoted to the history of rubber, the production of synthetic rubber, the chemistry of natural rubber, a comparison of natural and synthetic rubber and general chapters on utilization including; vulcanization. These subjects should be mentioned in a book of this kind, but the account is too detailed for the grower without being thorough enough to satisfy the technologist. It would be more appropriate if consumer requirements influenced the plantation processing of natural rubber, but this hardly arises in practice owing to the conservatism of the intermediate marketing organization and its isolation from producers and consumers alike. The author does not emphasize such deficiencies in market grading; for instance, although "the more obvious defects in smoked sheet" are quoted, of these six which largely determine prices only two on dirt content are actually important in the manufacturing of goods.
Apart from some production statistics there are no references later than 1958-understandable in a 1962 publication but nevertheless unfortunate. In particular, comparisons between natural and synthetic rubbers become dated very soon, because, although the properties of the former are virtually static, technologists eliminate defects in the latter. There have been some innovations in Hevea horticulture since 1958, e.g. rooting of leafy cuttings under mist, and green budding of young plants which has already become field practice on some estates. Again, Mr. Polhamus treats yield stimulation by synthetic growth substances as relatively unimportant and still experimental, whereas in fact the extra rubber per annum harvested in Malaya as a result of 2, 4-D and 2, 4, 5-T applications is conservatively estimated at 15, 000 tons. Nutrition gets a paragraph without mention of foliar analysis and the interesting if controversial ideas arising from it. Breeding is almost as lightly treated, the alleged out-crossing and low self-fertility in Hevea are taken as axiomatic, although new series of selfings have recently been attempted and found no less fertile than crosses; the low seed-set in monoclone areas is probably due to the uniformly level canopy reducing the amount of foliage exposed to the sun and the number of inflorescences. The rather vague paragraph "Breeding for high yield" gives the impression that probably only a few genes are involved, an idea which is hard to reconcile with the strong positive skew distribution of yield in seedling families. The claim that some interspecific hybrids have successfully combined Dothidella resistance with commercially satisfactory yields is not acceptable by modern Asian standards, where even higher yields must be attained to raise living standards and lower prices in face of synthetic competition. The value of this book lies in the history of the rubber-bearing crops, including the eventual failures such as Castilla, Parthenium, Taraxacum and Solidago. Only Hevea has emerged as a sustained commercial success after domestication outside its natural range in regions free from leaf blight. Probably no one else has had such long and close experience as the author of the attempts in the United States to develop alternative sources of natural rubber to the Hevea plantations in the Old World tropics. This detailed account of the American aspects leaves space for only a rather superficial treatment of the crop itself as judged by acreage, production and employment, but the comparative data on the botanical and geographical distribution of the rubber-bearing plants produce a stimulating study.
P.R.W.