294 INTRODUCTION 



In organization the cells of Spermatophytes do not differ essen- 

 tially from those of most Thallophytes. Excepting in the very 

 lowest forms, the cellular structures of Thallophytes are similar 

 to those of the Spermatophytes as a study of the unicellular 

 Thallophyte in Figure 25^ will show. This one-celled plant is 

 composed of protoplasm, which is the living substance, and a 

 wall, which encloses the protoplasm. The protoplasm, as in the 

 cells of higher plants, consists of nucleus and cytoplasm. The 

 nucleus, usually globular in shape, is en- 

 closed by a nuclear membrane and contains 

 one or more nucleoli (small globular bodies) 

 and chromatin (the chunky or granular sub- 

 stance scattered about in the nucleus). In 

 addition to nucleoli and chromatin, the nu- 

 FIG. 254. A cleus contains nuclear sap (water containing 

 one-celled Thallo- sugar> salts> and other su bstances in solu- 



phyte, Pleurococcus ^^ The ^ ^ t lagm Qut _ 



vulgans. n, nucleus . . 



showing nuclear sic * e * ^ ne nuc l eus > is vacuolate and has its 

 membrane, chro- outer border so modified as to form a mem- 

 matin, and a nucle- brane, which, unless the protoplasm is 

 olus. c, cytoplasm, shrunken, is tightly pressed against the cell 

 in which i there is a wftlL Water and solutions enter the oto . 

 large lobed chloro- ^ 

 plast x 800 plasm through this cell membrane by the 



processes of osmosis and diffusion. All of 

 these cellular structures have practically the same function here as 

 in the cells of the higher plants. In this particular unicellular plant 

 there is a chloroplast, which, like the chloroplasts in the food- 

 making cells of leaves, is a special protoplasmic body saturated 

 with a green pigment (chlorophyll), which enables it by utilizing 

 the sunlight to carry on photosynthesis, that is, to form sugar 

 from carbon dioxide and water. 



Although consisting of a single cell, this plant performs most of 

 the functions which the most highly organized plants perform, 

 but in a simpler way. In absorbing water and mineral elements 

 directly from its surroundings, it performs the function of roots. 

 In carrying on photosynthesis, it performs the function of leaves. 

 By dividing it gives rise to new individuals and thereby performs 

 the function of reproduction, which is the function of flowers. In 

 such a simple plant there is no function comparable to that of 

 a stem, for there are no distant parts, such as leaves and roots, 



