THE PAST HISTOllY OF PLANTS. 225 



The Cellulah Plants compriso many sorts, 

 from the simple one-celled types which float 

 freely in water, up to the relatively higli and 

 complex seaweeds, which produce large tleshy 

 fronds, and often display a considerable division 

 of labour between their various parts and organs. 

 Still, as most of them live in water, either fresh 

 or salt, and wave freely about in the liquid that 

 surrounds them, they have no need of an elabo- 

 rate system of conducting vessels, because every 

 part can drink in water and dissolved food- salts 

 from the neighbouring pond, sea, or river. Still 

 less have they any necessity for a woody stem, 

 which would only be a disadvantage to them in 

 stormy weather. Hence most of the cellular 

 2olants (with certain exceptions to be noted here- 

 after) are water- weeds ; while most of the 

 vascular plants (with other exceptions to be 

 similarly treated) are land plants. In particular 

 trees and shrubs, the highest forms of plant life, 

 are invariably terrestrial. 



Various successive stages of these cellular 

 plants may be briefly described in rough out- 

 line. First of all we get the simple one-celled 

 plant, the lowest type of all, consisting o* a 

 single mass of protoplasm, generally with 

 chlorophyll, surrounded by a cell- wall. Next 

 above these come the hair-like water-weeds, 

 which consist of rows of such simple cells, 

 placed end to end in single file, one in front of 

 another, like pearls in a necklace. These kinds 

 are many-celled, but each cell is here in contact 

 with two others only, one below, and one above 

 it. Thirdly, we get the flat leaf-like water- 



