BIONOMIC CHARACTERS OF PLANTS looi 



Thallophyta. 



Alg^. The lowest of the green algae (Protococcaceae) consti- 

 tute, with the diatoms, the Cyanophyceae, and the bacteria the 

 chief holoplanktonic plants, though many of them also have a 

 benthonic habit, being attached chiefly to other plants, or encrust- 

 ing stones, as in the case of some fresh water Cyanophyceae. They 

 are mainly inhabitants of fresh water, though diatoms are also 

 abundant in the sea. Protococcus itself is terrestrial. The Charo- 

 phycea, often classed with the green algae, secrete lime. This is 

 especially true of Chara, which occurs in fresh water lakes from 

 two meters down, and which is probably an important* factor in the 

 formation of fresh water limestones. (See ante, page 471.) The 

 majority of the green algae belong to the benthos, being equally 

 abundant in the marine and fresh water realms. The red algae 

 are benthonic and chiefly marine, though some species occur in 

 fresh water. Lime-secreting types also occur in this class, consti- 

 tuting the nullipores or coralline seaweeds which inhabit the littoral 

 district of the sea, chiefly in tropical regions but to some extent 

 also in temperate climates. They occur as jointed fronds of a red 

 color (Corallina), as crusts on other algae (Melobesia), or as ex- 

 tensive pink incrustations on stones or shells (Lithothamnion). 

 They are especially characteristic of depths between 15 and 35 (or 

 50) fathoms, where they constitute the "coralline zone" of the lit- 

 toral district. The brown algae are mainly marine benthonic types, 

 usually attached to a rocky substratum. The common brown algae 

 of the northern coasts (Fucus, Ascophyllum) have already been 

 referred to as clothing the rocky ledges and forming a substratum 

 for Hydrozoa, Bryozoa, sponges, etc., as well as other algae. Though 

 typically attached, some forms, like the Sargassum, will continue 

 to grow and multiply, even after they hav€ been torn from their 

 anchorage and carried into mid-ocean. The Sargassum is thus 

 typically pseudoplanktonic, as are also all the other algae attached 

 to it. Many of the brown seaweeds grow to great size. A familiar 

 example is the common kelp (Laminaria) of the Atlantic coast, 

 which generally grows to ten or more feet in length, while the 

 giant kelp of the Pacific (Macrocystis) is said sometimes to reach 

 a length of three hundred meters. The Laminarians are typical 

 of the littoral district down to fifteen fathoms, this interval being 

 known as the zone of Laminarians. The large species all grow in 

 the deeper portions of this zone, where they are anchored by 

 their "roots" to stones, shells, or other objects of support. They 



