The Lower Plants 57 



the primitive plant body. In such forms as that 

 shown in Fig. 4, C, the chlorophyll is mainly re- 

 stricted to small cells, whose number is increased 

 by the extensive branching of the filaments, so that 

 there arise dense tufts of small green cells, which 

 might very well be compared to a finely divided leaf, 

 and like the leaf, these may be fairly described as 

 special photosynthetic organs. The flat, leaf-like 

 body of the common sea-lettuce (Ulva) and many 

 other algae illustrate adaptation by which the amount 

 of green tissue exposed to the light is increased. 



Green Algae; Chlorophyceae. The primitive 

 fresh- water algae are none of them of very great 

 size, and for the most part they retain a simple 

 structure. Their color is usually a vivid green, un- 

 obscured by the red and brown pigments which dis- 

 tinguish most of the large seaweeds. These pure 

 green forms are therefore known as the " Green 

 Algae," or Chlorophyceae. While most of these are 

 confined to fresh water, some of them have migrated 

 to the sea, and in the altered environment have 

 undergone more or less marked changes. The most 

 striking of these green seaweeds are those known as 

 the Siphoneae, an assemblage of very peculiar green 

 plants distinguished by an almost complete suppres- 

 sion of cell division, so that the plants are composed 

 of a system of open, often very much twisted and 

 branched tubes, without any division walls. A few 

 of these curious plants occur in the cooler seas, but 

 to observe them in their fullest development one 



