MacDougal : Symbiosis and Saprophytism 523 



medio-cortex is i 5 to 20 layers in thickness and is composed of 

 cells with the radial diameter twice the axial. The outer cortex 

 consists of 3 to 5 layers of very thin-walled elements with the 

 tangential diameter greater than the radial. Both the medio- 

 cortex and the outer cortex are provided with intercellular spaces. 

 The epidermis is composed of flattened cells, irregular in outline, 

 with the lateral and inner walls pitted and the outer wall slightly 

 thickened and brownish in color. It is furnished with a large 

 number of stomata, with the motile guard cells of cfescentic form 

 (Plate 368, Figs. 5 and 6) containing starch. These and the large in- 

 tercellular spaces constitute a very efficient aerating sy.stem, and 

 makes the coralloid structure independent of the aerial shoot in 

 transpiration, and at the same time allows free access of atmos- 

 pheric oxygen. The apices of the rudimentary sheathing leaves 

 are soon converted into a number of blackish shreds and their 

 bases persist as wedge-shaped rudiments with no distinct function. 

 Clumps of large trichomes resembling root-hairs arise from papillae 

 which are infra-axillarv to the leaves. 



The fungus obtains access to the coralloid offset at quite an 

 early stage of its existence and constantly grows toward the elon- 

 gating .apex forming convolutions of pale gray shining hyphae 

 with numerous septae in the outer cortex. The growth of the 

 hyphae keeps pace with the offset in its slow growth, and they 

 extend forward to the shoulder of the blunt tip of each branch, 

 curving inward at this place toward the embryonic tissue. The 

 hyphae in the outer cortex remain active even in old mycorhiza 

 and may be designated as forming the permanent mycelium. The 

 permanent mycelium is thus in the shape of a sub-epidermal cylin- 

 der, and when new branches are formed the sheathing cylinder ot 

 the fungus is continued out in it. Branches from the apical por- 

 tion of the permanent mycelium penetrate the medio-cortex while 

 it is still in an undifferentiated condition, and these branches grow 

 and ramify with the development of the cortical cells until the 

 latter are almost filled with their dense convolutions. For some 

 unknown reason the development of the hyphal branches is great- 

 est immediately internal to the nodal trichomes in the medio- 

 cortex. The hyphal branches are generally cylindrical but occa- 

 sionally portions become swollen to twice the normal diameter, 



