85 



f 



this function it is in all respects similar to the foliage leaves of higher 

 plants. Jumelle ( 44 ) has shown that the variation in the function 

 of assimilation is much greater in lichens than in higher plants ; also 

 their adaptability to extremes of temperature. Assimilation varies 

 greatly in the different lichen-forms. There has been observed an 

 optimum of moisture ; increasing the degree of moisture above this 

 point reduces transpiration, as does also reduction below the opti- 

 mum. The extremes of temperature at which assimilation is still per- 

 ceptible are between 40 and 6oC. A temperature of 6oC. will, 

 however, destroy life, in some lichens in about one hour. 



The most important substance is no doubt lichenin or lichen-starch, 

 whose chemical formula is the same as that of cellulose and starch 

 (C 6 H 10 O 5 ). It occurs more or less plentifully in all lichens. In the 

 higher types it constitutes from 40% 1065% of the bulk. It forms 

 the principal constituent of the cell-walls. This substance absorbs 

 moisture very readily, but is comparatively slow to lose it. It also 

 becomes more or less gelatinized in different parts of the plant, es- 

 pecially in the cortical tissues and walls of the spore-sac. The 

 chemical behavior of lichen-starch is essentially different from 

 starch or cellulose ; it rarely gives the blue coloration with iodine, 

 as in spore-sacs and parts of the hypothecium. A strong solution of 

 potassium hydrate dissolves the cortical cell-walls of many lichens, 

 and causes all of them to swell considerably without producing any 

 coloration in the colorless hyphae. In the colored cell-walls this rea- 

 gent may produce brilliant effects, usually reddish or purple. Ny- 

 lander states that lichens also contain true starch-grains inter- 

 mingled with the tissue-elements, but this observation has more re- 

 cently been proven to be erroneous. 



Other important chemical constituents of many lichens are color- 

 ing substances deposited in the cell-walls in various parts of the 

 lichen, especially in the outermost layers of the cortical tissues, in 

 the upper ends of the paraphyses and in the hypothecium. Some- 

 times granules of coloring substance (usually acid crystals) are de- 

 posited on the outside of the hyphae, as in Solon'ua. The prevailing 

 colors are yellow, red, brown, dark-brown or black. The most im- 

 portant of these coloring substances is orchil (orseille, cudbear) which 

 occurs most plentifully in Rocclla tinctoria; in extracting it the 

 lichens are finely ground and leached in a solution of ammoniacal 

 potash until the lichen-mass takes on a purple color. The forma- 



