The lowest divisions of vegetable life may still be recognized 

 as ALG^;, LICHENES, and FUNGI ; and conveniently associated 

 together under the designation of THALLOPHYTES ; a thal- 

 lus, that is to say a form or forms of vegetation in which there 

 is no real distinction of stem and leaf being, in these plants, 

 with whatever exception, taken for characteristical. And there 

 is no doubt, notwithstanding the numerous and now startling 

 discrepancies of these vast groups, that they stand in close 

 natural relations to each other. 



Lichenes are reckoned as intermediate between the other 

 two Classes of Thallophytes ; but all the limits are uncertain. 



A lichen is (to speak only loosely) an aerial (*) Thallophyte, 

 vegetating only under the influence of moisture, and thus of 

 interrupted and slow (*) growth, but of indefinite duration (*) 

 characterized by certain green cells (gonidia ; gonimia) ; and 

 the organ of vegetation of which (thallus) is distinct (*) from 

 the organ of fructification (apotJiecium). 



The thallus of lichens is composed, to speak generally, of 1, 

 slender, more or less branched, loosely intertangled or closely 

 compacted cell-threads (filaments ; hyplice ; passing now into a 

 parenchymatous modification) which constitute the bulk of the 

 plant ; being distinguishable into a central, or medullary layer, 

 and an external, or cortical layer : and 2, of the just-named, 

 rounded or elliptical, green, or bluish-green, cells, which form, 

 for the most part, an irregular zone between the medullary and 

 cortical layers, and make what is known as the gonimous layer. : 

 These green cells (gonidia, which take on now, in certain con- 

 ditions of growth, a yellowish and even tawny coloration) owe 

 their colour to a chlorophyll-like matter called thallochlor ; as 



(*) Exceptions, at least apparent, but now also real occur; the rale 

 being however as stated. 



