PRINCIPAL GROUPS OF PLANTS. 71 



study of cancer tissue, is made up as follows : Malachite green, 

 0.50 Gm. ; acid fuchsin, o.io Gm. ; " Martius gelb," o.oi Gm. ; 

 water distilled, 150.00 c.c. ; alcohol (95 per cent.), 50.00 c.c. For 

 use with plant tissues the procedure is as follows : Wash in water 

 or alcohol, stain in the undiluted mixture 15 to 45 minutes, remove 

 excess stain in water, and decolorize in 95 per cent, alcohol to 

 which a few drops of hydrochloric acid have been added. For per- 

 manent mounts, clear with a carbol-turpentine mixture, remove 

 clearing solution, and mount in balsam. 



This stain is also valuable for staining spores which have been 

 allowed to germinate on the surfaces of leaves. In such cases 

 the killing and tissue-clearing mixture proposed by Duggar is 

 recommended, viz., consisting of equal parts of glacial acetic 

 acid and alcohol. In the study of the rusts, the best results are 

 obtained by the use of Durand's combination of Delafield's haema- 

 toxylin and eosin (Phytopathology, 1911, p. 129). 



LICHENS. 



General Characters. The Lichens are a peculiar group of 

 plants in that an individual lichen consists of both an alga called 

 a GONIDIUM and a fungus. These are so intimately associated that 

 they appear to be mutually beneficial, and such a relation is known 

 as SYMBIOSIS (Fig. 42). The Algae which may be thus associated 

 in the Lichens are those members of the Blue and Green Algae 

 which grow in damp places, as Pleurococcus, Nostoc, Lyngbya, 

 etc. (Fig. 42). The Fungi which occur in this relation belong 

 both to the Ascomycetes and Basidiomycetes, and it is on the 

 characters of the fruit bodies of these particular Fungi that the 

 main divisions of Lichens are based. The Fungi, however, are 

 not known to exist independently of the Algae with which they 

 are associated ; that is, the mycelia of the fungi will not live for 

 any length of time unless they come in contact with suitable 

 algae. In its development the fungus forms a mycelium which 

 encloses the alga, the growth of which latter is not hindered. The 

 two organisms then continue to grow simultaneously, forming 

 lichen patches. A section of a lichen shows a differentiation into 

 several parts (Fig. 43) : a more or less compact row of cells on 

 both surfaces forming two epidermal layers ; and an inner portion 



