Biology of Higher Plants — Anatomy and Physiology 231 



absorb a maximum of heat and light; (3) they are well supplied with 

 transportation vessels (veins ending in the mesophyll tissue) to transport 

 water and minerals to the photosynthesis apparatus and carbohydrates 

 away from that apparatus; (4) their transparent cuticle and epidermis 

 permit the entrance of heat and light but prevent excessive evaporation 

 of moisture; (5) their spongy mesophyll tissues with their air spaces 

 communicate through the stomata with the outside to permit the neces- 

 sary exchange of gases; (6) they possess a maximum of chloroplasts for 

 uniting carbon dioxide and water. In algae the water and carbon dioxide 

 enter through the cell membranes. 



Theories and Early Work on Photosynthesis. — Our present knowl- 

 edge of this phenomenon, like that of many of the great concepts, is due 

 to facts acquired by long, laborious experiments and observations by 

 many workers over a long period of time. Bonnet (1769) noticed bub- 

 bles of gas coming from living grape leaves immersed in water but no 

 bubbles from boiled water. Priesdey (1774) found that plants could 

 improve the air which had been rendered unfit by animals, hence sug- 

 gesting the exchange of carbon dioxide and oxygen between animals 

 and plants. Ingen-Housz, a Dutch physician (1779), showed that this 

 purification of the air was accomplished only by green plants and only in 

 light. Senebier (1782) showed that carbon dioxide was absorbed by 

 plants for nutritional purposes. De Saussure (1804) showed that plants 

 returned an amount of oxygen to the air which was about equal in 

 volume to the amount of carbon dioxide they had removed. He also 

 proved that the absorption and decomposition of carbon dioxide by a 

 plant resulted in an increase in weight of that plant. Boussingault 

 (1860-1890) carefully measured the carbon dioxide taken in by a plant 

 and the oxygen given off, thereby establishing their equality in volume. 

 Sachs (1862) concluded that starch grains in the green chloroplasts were 

 the product of photosynthesis in the presence of light. He also suspected 

 that there were intermediate products leading to the formation of starch. 

 He discovered that starch disappears from leaves at night and reappears 

 the next day. He proved that oxygen is a by-product of the process. 

 Von Baeyer (1870) formulated the formaldehyde hypothesis of photo- 

 synthesis in which he stated that small amounts of formaldehyde (CHoO) 

 were formed from water and carbon dioxide. 



The theories which have been proposed for explaining photosynthesis 

 are based on the supposed intermediate products of the photosynthetic 

 process. One theory is based on the supposition that formic acid and 



