138 PLANTS AND MAN 



and plants of high altitudes often develop more anthocyan than 

 plants of lower altitudes or warmer regions. 



The anthocyans play an important role in autumn coloration. 

 As the chlorophyll disintegrates and disappears, any sugar in the 

 leaves is converted into anthocyan pigments. The reds are 

 especially brilliant if a sudden frost, finding the sugars produced 

 by photosynthesis still in the leaves, is followed by bright weather, 

 since sunlight is necessary for the formation of these glucoside 

 pigments. Plants rich in sugars, like the maples, produce the 

 reddest autumn foliage. Ideal conditions for a predominance of 

 reds and scarlets therefore would include (/) a sudden early 

 frost, (2) followed by bright sunny weather {3) in a region made 

 up predominantly of maples. These conditions are usually best 

 fulfilled in New England, well noted for its gorgeous fall coloration. 



Other substances, related to the glucosides, are known as 

 TANNINS. These are astringent substances which form insoluble 

 compounds with tissues containing proteins; for this reason they 

 are valuable in tanning leather. Tannins are found in great 

 quantities in the leaves, bark and wood of oaks, hemlocks and 

 chestnuts. Though perhaps sometimes only a waste product, 

 tannins may be of some value as reserve foods in germinating 

 seeds, or as bitter substances in unripe fruits, preventing animal 

 attraction before the seeds are ready for dispersal. Their occur- 

 rence in bark suggests that they may act as antiseptic substances 

 preventing the growth of injurious fungi. It is the excess of tannins 

 in the leaves which gives the characteristic brown autumnal 

 color to beeches and oaks, which are particularly rich in tannins. 



Protein pigments are common among the algae. The Blue- 

 green Algae owe their color to the pigment phycocyanin, the 

 Red Algae to the pigment phycoerythrin (cf. p. 90). In 

 the case of the Red Algae, which live at considerable depths in 

 the oceans where only blue light is present, the phycoerythrin 

 acts as an auxiliary photosynthetic pigment by utilizing light 

 rays unabsorbed by chlorophyll alone. 



Plant Foods and Soils 



We have seen that photosynthesis produces the basic glucose 

 and that from this all the carbohydrates and fats are elaborated 



