50 BIOLOGICAL LECTURES. 



lose and touch each other at the base only, leaving wide 

 interstices between their outer extremities, which are occupied 

 by masses of hyphae. As a result of this arrangement, the 

 fungus appears to occupy the epidermal cells when a cross- 

 section of the root is examined, but such is not the case except 

 perhaps in Pterospora, Monoiropa, Hypopitys, and Sarcodes, 

 which I have recently examined as to this particular. Popiilus 

 (poplar), Querciis (oak), and the conifers furnish examples of 

 ectotropic mycorrhizas in which the fungus does not enter the 

 epidermal cells. 



In order to comprehend the other, or ectotropic type of 

 mycorrhiza, it will be necessary to recall that the typical struc- 

 ture of roots is one in which the central portion of the organ is 

 occupied by the stele, of which the fibro-vascular tissue is the 

 principal component in bulk, and that this central cylinder is 

 surrounded by a sheath composed of thin-walled cells of the 

 cortex, outside of which lies the cortex proper, made up of ten 

 to thirty layers of large parenchymatous cells. Covering 

 the root is the epidermal system of one to three layers, with 

 the outer walls of the outermost layer drawn out into tube-like 

 extensions — the root hairs. The hyphae of the fungus gain 

 entrance to the root when quite young by pushing into it from 

 the stem, or by piercing its tender epidermis. Once inside 

 the root, it generally shows a differentiation into three regions. 

 The hyphae which enter the root push forward through the 

 walls of the outer cortex toward the apex of the root, occupying 

 one to three layers of cells with hyphae which are fairly free 

 from convolutions or enlargements. This portion of the body of 

 the fungus may be termed the " vegetative mycelium." Numer- 

 ous branches from the vegetative mycelium are sent into the 

 middle region of the cortex, and these branches subdivide and 

 pierce the thin walls of the cortex, so that they may occupy 

 more than half of the cells. These internal branches appear 

 to be most delicately chemotropic. In some plants they are 

 attracted to the neighborhood of the nuclei, where they form 

 large vacuolated sacs, termed "vesicles," or agglomerations of 

 interwoven hyphae, which doubtless serve as organs of inter- 

 change between these two plants, as will be described further 



