Lecture IX —121— Structure 



against soil causes the cap to fit closely while friction of growth 

 causes sloughing of external cells. But contrast with the mycorrhizal 

 apex is decided, for in the mycorrhiza the root-cap is small, densely 

 filled with content and is closely covered over by a mantle of firmly 

 weft hyphae. At first white or light-coloured, it becomes dark with 

 age and the apical meristem finally ceases activity. In healthy mycor- 

 rhizae, said Mangin (1898), the root-cap instead of being lost is 

 conserved although invaded by mycelial filaments; and in part the 

 cells are detached from each other. The only change caused by pres- 

 sure of the mantle is a regular hemispheric form given to the root- 

 cap. The next year he pointed out that the mycorrhizal root-cap is 

 never exfoliated but persists throughout the life of the mycorrhiza 

 between the mycelial mantle and the more or less hypertrophied ex- 

 ternal cortical cells. And, as already noted, the apex may be covered 

 over with "root-hairs", as described by Gordon (1936) for broad- 

 leaved trees, by Muller (1886) for beech, and by Zach (1909) for 

 Sempervivum. 



The mycorrhizal apex in short-root of Corsican pine is neatly 

 illustrated by Aldrich-Blake in Oxford Forestry Memoirs : Details 

 of meristem, cap, and infected tissues are well-shown and are con- 

 formable to those of other species of pine. But in 1874 it was sup- 

 posed (by Janczewski) that gymnospermous roots have no cap nor 

 true epidermis although the primary cortex is exceedingly voluminous 

 over the root-apex, replacing the cap. MacDougal (1900) found the 

 root-cap little developed in Monotropa but many-layered in Sarcodes 

 and Pierospora: but in all cases the tip is covered with mycelial mantle 

 that crushes the cap-cells. 



Under favourable conditions of growth, as during a rainy season 

 that succeeds a drought, the apical meristem may renew its activity 

 and again cut off cells. As a result of pressure thus set up, mantle 

 covering the apex is split, exposing a new mantle that has developed 

 beneath, and the mycorrhizal short-root continues its growth in length. 



Renewed Growth: — This is such a common phenomenon it is 

 strange that it is not more generally remarked. Masui (1926) devoted 

 a paper to the subject, finding that "the mantle-clad root-apex of the 

 completed mycorrhiza in Abies firma (Form A and Form B), Abies 

 Mayriana, Alnns japonica and Finns densiflora can renew its growth 

 breaking through the mantle." The mantle splits in various directions, 

 hyphae grow out from the split and later cover the exposed root-tip. 

 Or, the quiescent meristem renews its growth after bark and mantle 

 have been sloughed off. 



