Lecture VI — 73 — Environment 



plants" in which mycorrhizae are located in a layer above the con- 

 ducting roots. For forest trees of central Europe, Klecka & 

 VuKOLOV (1937) state that mycorrhizae are developed most richly 

 and best in "the middle root depth". Scully (1942) found greatest 

 concentration of small roots in the A^ horizon and greatest numbers 

 of dead roots in a lower horizon. But trees may form mycorrhizae 

 at some depths in the soil : Pecan mycorrhizae may be formed at 30 

 inches depth (Woodroof, 1933), while mycorrhizal roots of Piniis 

 densiflora were found by Mimura (1933) at 10 m. depth. On shallow 

 infertile soils certain nut-trees have the roots confined to the upper 

 levels and the rootlets are almost entirely turned into mycorrhizae, 

 whereas on deep fertile soils the mycorrhizae are widely and deeply 

 distributed (Schuster, Stephenson & Evenden, 1944). Frank 

 (1887) had stated that in German forests the mycorrhizae occur in 

 the uppermost 1.5 cm. of humus while at lower depths there are 

 fewer although they may be found at ^ m. depth. Yeates (1924) 

 also found, in the New Zealand Podocarpus, that the roots are mostly 

 at the surface in an organic layer, a condition which, he said, is pos- 

 sible only in a rain forest. In Finland, Laitakari (1934), in an 

 elaborate study of root development in Bctiila, found that horizontal 

 roots occurred at depths of 2.8 cm. to 31.1 cm.; on moranic soils the 

 depths were greatest (av. 20 cm.) while on water-logged soils they 

 were least (ca 8 cm.). Vertical roots penetrated to depths of over 

 2 m., being deepest in clay. Fagus in England, according tO' Harley 

 (1937), shows variations in root systems with depth of soil: In 

 shallow soils on chalk the whole substrate is colonized while in deep 

 plateau soils the roots are fairly evenly distributed in upper layers 

 of mineral soil. In podsols and semi-podsols, fine roots are restricted 

 to litter and humus layers. 



Herbs and dwarf shrubs apparently have superficially placed 

 mycorrhizae. Calluna root system (Rayner, 1911) is confined to the 

 first 12 inches of soil; while Burgeff (1932) stated that hemi- 

 saprophytic organs of orchids are found in the uppermost layers of 

 soil. 



Soil Texture: — Effect of soil texture on root development was 

 neatly shown by Ter-Sarkisow {cf. Kirchner, 1908) for Pinus 

 sylvestris, 4 month old seedlings in pots showing the following root 

 development : 



Number Length 



Sand 363 713 



Loam 181 420 



Humus 54 179 



