212 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



in root-form. The rapid progress in recent years of soil science 

 gives hope that more research will be devoted to the acquisition of a 

 knowledge of the special relationships of the root to the soil. I 

 believe that it will ultimately be found that roots have a more specific 

 relationship to their environment than that shown by the shoot, and 

 the adaptations which will be discovered of the roots to soils of 

 varying water content and of varying chemical constitution will be 

 specifically defined. 



To illustrate the special attunement that is observable in soil-roots 

 in relation to physiological drought, I will continue my reference to 

 features in plants growing in peat which, as I have said, is so typically 

 a soil of physiological drought. 



The first character deserving notice in the roots of peat plants is 

 the ejdstence of a fungus in association with the root to form 

 mycorrhiza. Such an association is nowadays familiar to all horti- 

 culturists in connexion with the raising of orchids from seed. Like 

 these young orchids, peat plants are attuned to the condition, and 

 one must regard it as an important and fixed character in the 

 organism. But remember, mycorrhiza is not specific. Any plant may 

 form it, although certainly not the same fungus is engaged in every 

 case. Most gardeners come in contact with this, although it may not 

 be recognized by them. Thus, when plants are grown in tubs, and 

 the surface of the tub in contact with the soil begins to rot, the roots 

 of the plant will be found penetrating the wood and always having 

 the character of mycorrhiza. No matter what be the plant grown 

 the mycorrhiza develops in those conditions, and it is this character 

 which calls for special care in the work of re-tubbing tub-grown plants 

 when their roots have filled the tub. Eough removal of the wood of 

 the old tub will take away also the mycorrhiza, and the plant will 

 suffer from want of all that is involved in the presence of mycor- 

 rhiza. The disasters that so often follow separation of a plant from 

 a tub in which it has long grown are explained by this. 



We have in this a clue to some of the value of mycorrhiza to the 

 peat plant. One may suppose that the initial invasion may have been 

 guided by questions of nitrogen supply — the fungus taking from the 

 soil organic nitrogen and passing it on to the host — yet it is* im- 

 possible to separate the water problem from this, for the fungus aids 

 in the intake of water from the retentive hnmus, and the develop- 

 ment of the mycorrhiza is an adjuvant to water absorption and may 

 be regarded as one of the adaptations to help in overcoming physio- 

 logical drought at the point of intake of the supply. Nor is the 

 function unusual. For in many plants of physical drought, where 

 mycorrhiza is more developed than it is in peat plants, and forms a 

 sheath to the root, it performs the whole work that is commonly doncj 

 by the root-hairs. 



But that is not the whole story of mycorrhiza and its value to the! 

 peat plant. Although discovered so long ago as the early years of 

 last century, its significance is only now becoming recognized and| 



