474 BOTANY 



sometimes no starch can be detected in the chloroplast, and instead 

 we find oil or sugar. It is not impossible that the first substance 

 developed is sugar, or there may, perhaps, be a simple carbohydrate 

 like formic-aldehyde (COH), which is volatile and may be easily con- 

 verted into more easily detected compounds, as sugar, starch, or oil. 

 The development of starch in the chloroplast under the influence 

 of light can readily be observed in many green plants. Within a 

 short time after the green cells are exposed to the light, the presence 

 of small starch grains may be detected in the chloroplasts. Removed 

 from the light, the starch soon disappears, A very simple demon- 

 stration of the development of large quantities of starch as the result 

 of photosynthesis is seen, where part of a leaf is covered with an 

 opaque shield, such as a strip of tinfoil, and the leaf then exposed 

 to the sun for several hours. If the leaf is now treated with alcohol 

 until the chlorophyll is entirely removed, and then with a solution of 

 iodine, the portion of the leaf exposed to the sunshine will show the 

 characteristic blue colour, indicating the presence of starch in the cells, 



while the shaded portion 

 will simply show the yel- 

 low tinge of the proto- 

 plasm stained by the 

 iodine solution (Fig. 453). 

 While sugar may re- 

 place starch as the first 

 demonstrable product of 

 carbon- dioxide -assimila- 

 FIG. 453. A leaf which has been covered with a tion it is not entirely 

 stencil plate leaving only the letters exposed to ^ { ^^ fch R f d 

 the action of light. Ihe leaf has had the chloro- . 



phyll removed with alcohol, and was then treated m the chloroplasts of 

 with iodine, which colors the exposed parts blue, some of the Monocoty- 

 on account of the starch formed there, under the T^J^ /, C4- ~K<- ;\ 

 action of light. (After PFEFFER.) led ns ( e & Strelltzia), 



and in such Algae as 



Vaucheria, is the direct product of photosynthesis, and the same 

 is true of the proteids generally assumed to be of this nature. In 

 these cases, it is not impossible that the cytoplasm may also be 

 concerned in the manufacture of these substances, which perhaps 

 arise chemosynthetically, and not photosynthetically. 



No carbon compounds, except CO 2 , can be assimilated photosyn- 

 thetically. In certain succulent plants (Crassulacese, Cacti, etc.) 

 there are produced in darkness certain organic acids (malic, isomalic, 

 and oxalic). These are decomposed when the plants are exposed to 

 the light, with evolution of oxygen and production of starch, but 

 this does not seem to be a case of true photosynthesis. The process 

 seems to be an oxidation of the acid, with a corresponding evolution 

 of C0 2 , which is then made use of by the chloroplast. This peculiar- 



