ASSIMILATION OF CARBON 21 



dark, bluish-red solution shows an orange-yellow fluorescence. It crystallizes 

 from salt solutions in hexagonal red crystals.' 



Phycocyanin,^ the blue pigment of the blue-green algse, Cyanophycese, is 

 likewise of protein nature; it is soluble in water and glycerine but insoluble in 

 ether and alcohol; its crystals are indigo blue in color. 



The brown algae contain a pigment, phycophcdn,^ which is easUy soluble in 

 water; in concentrated solutions it is dark reddish-brown.* 



Engelmann' studied the absorption spectra of bright-colored leaves of va- 

 rious plants, and Stahl* investigated the biological importance of their coloring.' 



§5. Influence of Light upon the Decomposition of Carbonic Acid by Plants. 

 ■ — An acquaintance with the properties of the different rays of the sun's spectrum 

 (Fig. 12) is prerequisite to an understanding of the researches devoted to this 

 subject. Only the central part of the spectrum, approximately that portion 

 lying between lines A and E, is visible to the human eye; on either side are in- 

 visible rays, infra-red to the left a^nd ultra-violet to the right. Of the visible 

 rays, the yellow are the brightest, the brightness reaching a maximum at line 

 D and decreasing to zero beyond A and H. Brightness does not, however, 

 represent the character of the rays, but only that of the human eye. The en- 

 ergy maximum in the prismatic solar spectrum is usually shown as falling in the 

 region of the infra-red, as in Fig. 12. Nevertheless, recent work upon the dis- 

 tribution of heat in the ordinary diffraction spectrum of sunlight shows the 



1 Molisch, Hans, Das Phycocyan, ein krystallisirbarerEiweisskorper. Bot. Zeitg. 53 : 131-135. 1895. 



2 Schurt, Franz, Ueber das Phycophsein. Ber. Deutsch. Bot. Ges. 5: 359-274. 1887. 



3 Engelmann, TIi. W., Die Farben bunter Laubblatter und ihre Bedeutung fur die Zerlegung der 

 Kohlensaure im Lichte. Bot. Zeitg. 45: 393-398, 409-419. 42S-436, 441-450, 457-469. 1887. 



* Stahl, E., Ueber bunte Laubblatter. Ein Beitrag zur Pflanzenbiologie. II. Ann. Jard. Bot. Buitenzorg 

 13: 137-216. 1896. 



' On phycoerythrin, see Haas and Hill, 1913. [See note 6, p. 3]. The best study of this 

 pigment is that of Hanson. (Hanson, E. K., Observations on phycoerythrin, the red pigment 

 of deep-water algae. New phytol. 8: 337-344. 1909.) — Ed. 



* But it seems to have been shown that there is no such pigment as phycophaein in the living 

 cells, this being a post-mortem product of the decomposition of a colorless chromogen. The 

 brown color of the brown algae is at least partly due to the presence of carotin. In this con- 

 nection see the following: Molisch, Hans, Das Phycoerthyrin, seine Krystalisirbarkeit 

 und chemische Natur. Bot. Zeitg. 52 : 177-189. 1894. Idem, Das Phycocyan ein Krystal- 

 lisirbarer Eisweisskorper. Ibid. 53 : 131-135. 1895. Idem, Ueber den braunen Farbstoff 

 der Phaeophyceen und Diatomeen. Ibid. 63^: 131-144. 1905. Tswett, M., Zur Kenntnis 

 der Phaeophyceenfarbstoffe. Ber. Deutsch. Bot. Ges. 24 : 235-244. 1906.- — Ed. 



' The anthocyanins, or anthocyans, are other pigments that may be mentioned here. They 

 occur very commonly in flowers, leaves, stems, fruits, and even in roots, giving them a red, 

 blue or purple color and frequently masking the green of the chlorophyll in leaves. They are 

 red when acid and blue when alkaline. The color of red apples and many other fruits, of 

 many red, blue and purple flowers, of the beet-root, of red cabbage, of yojing leaves of many 

 plants, and of the bronze-colored leaves of the copper beech, are due to the presence of these 

 pigments. They are often present along with chlorophyll, as in the case of red cabbage and the 

 copper beech, and stiU other pigments frequently accompany them. They are soluble in water, 

 alcohol and ether, and the color of the solution alters from red to purple or blue as the reaction 

 is altered from acid to neutral or alkaline. For further information see: Haas and Hill, 1913. 

 [See note 3, p. 6.] West, Clarence J., Plant pigments: The chemistry of plant pigments other 

 than chlorophyll. Biochem. bull. 4: 151-160. 1915. — Ed. 



