MYCORRHIZA IN RELATION TO FORESTRY 295 



lings raised in them. At the end of a year's growth these have larger shoots 

 with longer needles than those of controls raised in the same compost lacking 

 humus treatment, and give evidence of more vigorous growth, especially 

 towards the end of the season. The root systems are remarkably different : 

 those of controls poorly branched with scanty development of sub-lateral 

 roots ; those of treated seedlings well branched with abundant sub-laterals, 

 all of which become mycorrhizas. The beneficial effect upon gro\yth 

 continues during the second year : treated seedlings start growth earlier, 

 have longer needles and grow more vigorously. 



These beneficent effects are evidently due to developmental changes 

 controlled by factors operating within comparatively narrow limits since 

 they are readily influenced by variation in the experimental treatment, as, 

 for example, the date of sowing. They are well marked in seedlings from 

 sowings up to the end of May, but diminish in those from later sowings. In 

 July sowings there is an improvement in the root systems, but mycorrhiza 

 is not formed during the first year, and improved growth of the shoot is not 

 observable. 



(2) It is evident that the improved growth observed depends upon 

 biological causes of a reciprocal kind. Thus, seedlings of Corsican Pine and 

 Maritime Pine raised on Wareham soil without treatment are already in an 

 unthrifty condition at the end of one year's growth. Such seedlings do not 

 benefit from appropriate humus treatment in the spring of the second year. 

 Whether they may do so eventually is a matter of some biological interest, 

 but is clearly not one of practical importance. In general, it is clear that 

 the effects produced vary directly with the technique used. It is, therefore, 

 important that a suitable technique should be devised in using methods of 

 humus inoculation for promoting mycorrhiza formation. 



These facts provide a clue to the irregular and often confusing results 

 yielded by empirical humus treatments in the hands of foresters. 



(3) The marked stimulation to root development and mycorrhiza formation 

 and the improved growth that follow suitable humus treatment do not 

 depend upon proved inability of Corsican Pine and Maritime Pine to form 

 mycorrhiza with fungus species present in the Wareham and Ringwood soils. 

 Both species are capable of forming mycorrhizal associations ^yith indi- 

 genous soil fungi — Maritime Pine with comparative ease, Corsican Pine 

 apparently with some diflSculty. Whether the improvement effected in this 

 respect by the introduction of small quantities of native humus is due to the 

 introduction of more favourable fungus associates or to other causes is not 

 at present known with certainty. 



The significance of mycorrhiza in relation to forestry problems offers a 

 stimulating field for research. For various reasons it has been judged 

 expedient to concentrate attention on certain aspects of the problem likely 

 to yield results of immediate practical interest. 



It is clear, however, that fundamental aspects such as the physiology of 

 the nutritive relations between roots and mycelium and the exact nature of 

 the beneficent effects observed call for intensive laboratory research. 



All aspects of the problem are bound up with a clearer understanding of 

 the conditions controlling decomposition changes in organic soils, calling for 

 the close co-operation of the soil chemist. 



The grant of £25 provided by the British Association has been fully 

 expended, and the Committee do not ask for reappointment. 



In presenting this final report they desire to record their appreciation of 

 the services rendered to the work in its initial stages by the three grants 

 provided by the British Association. 



The continuance of the work on a somewhat enlarged scale has now 

 become possible, owing to a grant provided by the Forestry Commission. 



