DEOUGHT AND GARDENING. 



215 



relaimg to the cultivation of any special mycorrhizal fungus on lime 

 substratum, we do know that certain soil fungi, which may therefore 

 be mycorrhizal, will not grow upon lime soil. 



Then, whatever be the function of the excessive acid mucilage, 

 the neutralizing effect of the lime, if it reach the mucilage, should 

 interfere with its action. 



Further, seedling EhododendroDS watered with water rich in lime 

 are deficient just in those features— mycorrhizal fungus and mucilage 

 secretion. 



Again, in the two European alpine forms of Ehododendron, which 

 v/e may in general terrhs speak of as the peat form — Rhododendron 

 ferrugineum — and the limestone form — Rhododendron hirsutum — the 

 actual root structure differs significantly in the particular point of the 

 mucilage-forming layer. The peat plant has a large one, the lime 

 plant an indifferently small one. 



All this evidence seems to be in the direction of showing that the 

 cause of the refusal of Ehododendrons to grow in soil containing lime 

 may be that lime prevents the proper development of mycorrhiza as 

 well as the formation or action of the root mucilage, the two adapta- 

 tions — particularly the latter— which the plant has apparently acquired 

 in relation to conditions of water and nitrogen in peat. 



I must take note hereof this, however: modern discovery seems 

 to suggest that all Ehododendrons are not attuned so specially as we 

 have hitherto supposed the plants of the genus to be to peat and its 

 drought. We know that Rhododendron ferrugineum of the Alps, 

 whilst essentially a humus plant, is indifferent to this extent, that it 

 grows upon limestone if there be a superlayer of humus, and in 

 practice we know that Rhododendron hirsutum, though a lime plant, 

 will grow in well-aerated peat, u-i^r. Forrest, who has done such 

 magnificent exploration work, in Yunnan, tells me that Ehododendrons 

 grow luxuriantly on the limestone mountain ranges there — roots pene- 

 trating far into the crevices, of the limestone rocks — very different 

 this from our Himalayan forms. If these reputed limestone forms 

 confirm the expectation which we may form, we ought to obtain from 

 them valuable information bearing on the root-relationships of plants 

 to soils, but beyond that the gain accruing to horticulture in a group of 

 Rhododendrons, either true species or races raised by crossing, which 

 would be not lime-haters, but actually lime-lovers, requires no 

 emphasizing from me. 



/ The recognition by the gardener of the indications in shoot and 

 root given by plants attuned to such permanent physiological drought 

 as are peat plants should be a help to him in solving his problem of 

 correct watering. In essence, that is regulation in relation to the 

 air-supply in the soil. How vital is this is shown by an interesting 

 feature in the structure of the roots of many of these peat plants — • 

 namely, the presence of a bright red root -tip of variable size (see fig. 71). 

 This tip — oxidation tip we may call it — is present in plants of so many 

 families that it must have a general significance. It appears to be the 



