116 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



seen on digging up a large clump of soil from which is growing a 

 patch of Yellow Rattle, Eyebright, or Cow-wheat ; set the mass in 

 a large basin of water, wash it gently, and clip out the roots of the 

 parasite and the host-plant (a Grass) at points where they are in 

 contact. Sections will show that the parasitic root forms a swollen 

 mass of tissue, from which there proceeds a haustorium containing 

 a strand of xylem vessels, much as in the Dodder. 



137. Endotrophic Mycorhiza of Bird's-Nest Orchid. Dig 



up a plant of Neottia, or at any rate remove portions of the thick 

 fleshy roots. In transverse sections of a root, note (a) the central 

 cylinder with its alternating xylem and phloem strands ; (b) the 

 thick cortex ; with the fungus-zone near the periphery, within 

 (c) the epidermis. 



Examine the fungus-zone more closely, and note that it is usually 

 in three layers of cells. In the outer layer (that immediately within 

 the epidermis) and in the inner layer of the fungus zone, the fungus 

 hyphae are slender and usually clustered round a central mass of 

 proteid in the cell ; while in the middle layer the fungus hyphae 

 are stouter and practically fill up the entire cell cavity. As a rule 

 the fungus, after infecting the three outermost layers of the cortex, 

 thrives only in the middle one of the three layers, while in the outer 

 and inner layers its growth is checked by the living protoplasm of 

 the cortex-cells, which absorb the food-materials provided by the 

 fungus. 



138. Exotrophic Mycorhiza of Beech, etc. Dig up a Beech 

 seedling, rinse in water, and note that the rootlets bear (a) fine 

 white fungus- threads clearly not root-hairs, since they branch 

 freely and arise from (b) a fungus mantle of interwoven threads 

 (hyphae) covering the surface of the root ; (c) masses of humus 

 attached to the fungus threads. Cut and examine transverse and 

 (more instructive) longitudinal sections of the root. A similar 

 fungus mantle may be found on various other humus-loving plants, 

 e.g. Heather. 



139. Laticiferous Tissue. The latex ( 89) found in the 

 various plants is contained in special tissues, of which the chief 

 forms are ( 1 ) latex vessels or syncytes, produced by the fusion 

 of original separate cells ; (2) latex cells or coenocytes, which 

 branch but do not fuse or anastomose. Latex vessels occur in Dan- 

 delion and some other Composites, Campanula, Chelidonium, Poppy, 

 etc. ; latex cells in various Euphorbiaceae, Apocynaceae, Ascle- 

 piadaceae, etc. 



(1) Latex Vessels. Cut (a) transverse sections of the root of 

 Dandelion, (6) tangential longitudinal sections passing through the 

 phloem ; treat some with potash, others with alkanna, others with 

 potassium dichromate, and mount in glycerine. In (a) note the 



