34 MANUAL OF BOTANY 



Class I.— ALG^. 



As we have already seen, there exists m this group a won- 

 derful variety of forms, ranging from single cells of microscopic 

 dimensions to immense plants showing both morphological and 

 anatomical differentiation. They are mainly aquatic in their habit, 

 being found in both fresh and salt water. Many of the humbler 

 of them occur not in water, but in moist situations, such as on 

 rocks, banks, tree trunks, &c. Some few forms are epiphytic, 

 living attached to other Algae by filamentous or more massive 

 outgrowths, which are embedded in the tissues of their hosts. 

 Though epiphytic they are usually not parasites. 



They all contain colouring matters of some kind ; chlorophyll 

 is always present, but in some it is masked by the presence of 

 other pigments in the chromoplastids. We have blue-green, 

 green, brown and red Algae, and the distinguishing colouring 

 matters serve as a primary basis of classification in the group. 



The forms that live in fresh water are chiefly green, though 

 there are among them a few red forms, and many which are 

 blue-green. A large number of microscopic forms, belonging to 

 the Diatoms, also inhabit fresh water. These are olive -brown 

 in colour. 



The massive forms include representatives of all four colours : 

 their distribution bears a certain relationship to their hue. 

 Thus the seaweeds which are found near the limit of high-water 

 are green ; between high and low-water marks the olive-brown 

 forms are more prominent, while the red forms are also sparsely 

 represented in this area. Beyond low-water mark the olive- 

 green gradually give place to the red forms, and as the dex3th 

 increases the latter become predominant. They are seldom found 

 beyond a depth of 250 to 300 feet, so that the massive flora 

 occupies a belt along the shore, extending from that depth to the 

 limit of high water. Some forms are attached to a substratum, 

 others float freely in the water. 



The form which the adult plant exhibits is, as we have seen, 

 the gametophyte, either actual or potential. The sporophyte 

 rarely occurs, and then only in the higher members of the green 

 and red forms. 



In histological differentiation the Algae are all very simple 

 as compared with the higher plants. Many of them are 



