64 COMPOUND ORGANS OF PLANTS. 



placed at the time that its vegetative powers are called into 

 action. 



75. Algse have been considered by some to be lichens 

 growing in water, and in the Synopsis of the Lirhenes, re- 

 cently published by Edward Tuokerman, A. M., of Boston, 

 lichens are defined to be "perennial, aerial algae." It is true 

 that the hard, dry, scaly, and almost indestructible thallus of 

 such species of lichens as grow on the surface of exposed 

 rooks, old fences, or the bark of trees, is very unlike the soft, 

 easily decomposed, and leaflike frond of the algse or sea-weed ; 

 nevertheless, when lichens grow in moist situations, they 

 approximate towards the algte in point of organization, and 

 in some cases, as in the genus GoUema, it is difficult to fix 

 the group to which they belong. Certain it is that this tribe 

 of plants deserves the closest investigation of the physiologist. 

 In them we have the simplest expression of the laws of vege- 

 table life, and they are probably the best starting-point in the 

 study of the vegetable creation. 



76. Hitherto, the plants investigated have been stemless, 

 leafless, and rootless, presenting to the eye the appearance of 

 powdery, filamentous, or flat foliaceous expansions of vegetable 

 matter, having the reproductive cells imbedded on the surface 

 or restricted to the extremities. These plants, which are 

 without any distinct root, stem, or foliage, have been called 

 collectively Thallophtes, because they consist of a thallus or 

 bed of vegetable matter; and the name is very appropriate, 

 for the greater part of these rootless, stemless, and leafless 

 vegetable bodies consist of a collection of cells commonly 

 aggregated so as to make up a stratum or bed of vegetation 



