188 



General Biology 



taken in is dissolved in the water in which it is readily soluble. While 

 the exact steps in the process of formation of carbohydrate foods from 

 these substances are not yet clear, the essential facts are well established. 

 The carbon dioxide and water are partially or completely reduced to 

 their elements, which immediately recombine to form a monosaccharide 

 sugar (probably dextrose) with the freeing of oxygen. These two 

 processes are represented by the reaction 6C0 5 +6H 2 = C 6 H 12 6 +60' 2 . 

 The oxygen is given off into the atmosphere through the cell wall. The 

 sugar is the primary food of the plant, being the principal material 



used in the synthesis of other 

 foods and in the processes of 

 metabolism. When it is pro- 

 duced in excess of immediate 

 requirements, a further re- 

 action takes place by which 

 some of the water is eliminated 

 and the sugar is 'condensed' 

 into starch. This reaction is 

 n(C 6 H 12 O c ) = (C 6 H l0 ,O 5 )n+ 

 n(H 2 0)." 



Starch is deposited in the 

 chloroplast as granules, or 

 "starch grains," which form a 

 reserve food supply for the cell. 

 Starch grains soon disappear in 

 green plants kept in darkness 

 only to reappear after the plant 

 has again been in the light for 

 a considerable period of time. 

 In some plants, as in Vaucheria 

 (Fig. 86), the excess food is 

 stored in the form of a fat or 

 oil, but it is probable that here, 

 too, sugar is the first food 

 formed. 



The process by which car- 

 bohydrates are manufactured 

 in green plants is called photo- 

 synthesis. Its essential fea- 

 tures may be summarized as 

 follows : "The materials used 

 are carbon dioxide and water; 

 the energy is obtained from 

 sunlight absorbed by chlo- 

 rophyl; the chloroplast by the 





Fig. 86. 



I. Asexual Reproduction of the Green Felt 



(Vaucheria.) 



A, formation and discharge of the large, many- 

 ciliate zoospore from the terminal sporangium; B, 

 the zoospore showing the ciliated surface; C, sec- 

 tion through the surface of the zoospore showing the 

 pairs of cilia above the nuclei and the layer of plastids 

 beneath; D, germination of zoospore; E, young plant 

 of Vaucheria, the two filaments having arisen at op- 

 posite ends of the zoospore, one having developed an 

 organ of attachment or holdfast h; F, a group of 

 plastids, the lower in process of division. (A, B, 

 after Gotz; C, after Strasburger; D, E, after Sachs.) 



II. Sexual Reproduction of the Green Felt 



(Vaucheiia.) 



A, Vaucheria sessilis; o, oogonium; a, antheridi- 

 um; os, the thick-walled oospore, and beside it an 

 empty antheridium; B, Vaucheria geminata, a short 

 lateral branch developing a cluster of oogonia and a 

 later stage with mature oogonia o and empty an- 

 theridium a; C, sperms; D, germinating oospore. 

 (From Bergen & Davis "Principles of Botany" by 

 permission of Ginn & Co., Publishers. C, after 

 Woronin; D, after Sachs.) 



