Simple Plants 



205 



THALLOPHYTES 



Thallophytes. These plants have a mere plant body without a true 

 stem, roots, or leaves, though there may be parts that resemble stems, 

 roots and leaves. They may be very fragile, as are some of the thread- 

 like green Spirogyra (Fig. 93), (also called pond-scum and frog- 

 spit), commonly found in fresh-water creeks and ponds, or tough sea- 

 weeds, like the brown kelp many feet in length. The cells usually grow 

 end to end. 



Cytoph 



Fig. 93. 



The band-like chloroplasts extend 

 in a spiral from one end of the cell 

 to the other. In them are imbedded 

 nodule-like bodies (pyrenoids), and 

 near the center of the cell the 

 nucleus is swung by radiating 

 strands of cytoplasm. (After Stras- 

 burger.) 



. As was seen when living organisms 



were discussed, there are no hard-and-fast 



'hhroplad rules by which one may classify anything. 



There are some Thallophytes which really 



-JVuc/eus have stem-like and leaf-like structures, but 



Pyrenoids the classification originally based on struc- 

 tures, must now be thought of more from 

 a functional or life-cycle point of view. All 

 thallophytes are alike in having a more or 

 less simple life-cycle, so this must serve us 

 as a basis. 



are red, Rhodophyceae ( 

 Myxophyceae ( 

 Spirogyra ( 



The various algae (Fig. 94) are named 



after some distinctive characteristic ; thus 



those which are green are called Chloro- 



phyceae ( ) ; those which 



) ; those which are slimy, 



), etc. 



) is in turn a representative of the 



Chlorophyceae. The cells are elongated and attached end to end. There 

 are spirally arranged bands (chromatophores or chloroplasts) which 

 contain chlorophyl. The number of these bands and the method of 

 coiling depend upon the species to which each belongs. The cytoplasm 

 is rather thin and lies next to the cell-wall, while fine threads of it 

 extend to the nucleus. The special centers in the chloroplasts, where 

 starch is stored, are called pyrenoids ( ). Nearly 



98 per cent of the cell is water, yet the two per cent remaining can 

 perform every one of the four vital processes. The bubbles often seen 

 are filled with oxygen which is a waste product of photosynthesis. 



Reproduction takes place in two ways, either by the individual cell 

 dividing at right angles to the length of the cell, or after two individuals 

 have conjugated. The latter is seen when two plants lying close together 

 send out projections (Fig. 95) which unite, forming bridges through 

 which the cytoplasm of one plant mixes with another. In fact, these 

 two cells may unite so thoroughly that they become one, becoming 



