436 



HISTORICAL GEOLOGY. 



The other principal divisions of the Acrogens are the following : — 



(1) Equiseta, or Horse-tails. — The existing species have hollow-jointed, slender stems ; 

 the leaves arranged in whorls at the nodes ; and the cone-like fructification at the ends of 

 the stems. Ancient species grew with stout trunks to a height of 30 feet or more. 



(2) Lycopods, or Ground-Pines. — The Lycopods have many leaves, with the habit of 



484. 



485. 



486. 



Rhizocarps. — Fig. 484, Pilularia globulifera, with fructification; 485, Marsilea quadrifolia, with an enlarged 

 view of the nut; 486, Salvinia natans, part of plant. All half the natural size, Luerssen. 



a Spruce or Pine ; they are small plants now, but in the Coal era grew up as trees, 30 to 

 90 feet in height. 



(3) Ferns. — Modern Ferns sometimes make trees 20 to 30 feet high. 



2. The Lower Cr3^togams. 



The Lower Cryptogams consist of cellular tissue alone, 

 are : — 



The principal groups 



1. Mosses. — Green, terrestrial plants having delicate leaves along the slender stems ; 

 limited to a few inches in the height of the living part of stems. Closely related to the 

 Mosses are the Hepaticoe, or Liverworts. 



2. Lichens. — Dry plants, of gray, brown, and black colors, making fronds without 

 leaves, which spread over the surfaces of rocks, the outer bark of trees, etc. 



3. Fungi. — Succulent plants, gray to brown in color, and never green ; without 

 foliage ; grading down to Molds, which consist of strings and groups of cellules, and to 

 Bacteria and other microscopic, free-swimming, unicellular kinds. 



4. Algae, or Seaweeds. — The water-plants are green to brown in color, and contain 

 more or less chlorophyl. They graduate downward from ordinary seaweeds to micro- 

 scopic, free-swimming, unicellular kinds. Of like grade with the unicellular species are 

 other kinds having the form of threads or groups of threads, each thread consisting of a 

 series of cells. The lowest groups include the species of Protococcus, of which P. nivalis 

 is red and gives the red color to the snow or ice in some Alpine regions. The Diatoms 



