204 



GENERAL BIOLOGY 



THALLOPHYTES 



Thallophytes These plants have a mere plant body, there being no 

 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. 



As has been seen by this time, when 

 living organisms are discussed, there are no 

 hard and fast rules by which one may 

 classify anything. There are some Thallo- 

 phytes which really have stem-like and 

 leaf-like structures, but the classification 

 originally based on structures, 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. 

 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 



chlorophyll Unit 



pyrenoids 



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 

 strand* of cytoplasm. (After Stras- 

 burger. ) 



are red, Rhodophyceae ( 

 Myxophyceae ( 

 Spirogyra ( 



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

 are spirally arranged bands (chromatophores or chloroplasts) which con- 

 tain 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 2 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 to- 

 gether 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, 



