282 PLANT PHYSIOLOGY 



An annual translocation from the leaves back to the stems oc- 

 curs in the fall before the leaves die and fall off. Since all material 

 which goes off in the leaves is sheer waste, the plant conserves as 

 much as possible by translocating large amounts back into the stem 

 and the organs below the leaves. Combes and Kohler (1922) have 

 calculated that when leaves die in the fall 45% of the carbohy- 

 drates remains in the leaf (cellulose cell walls, etc.), that 35% is 

 burned up in respiration after the chlorophyll has ceased to func- 

 tion, and that 20% is returned to the stem. Much of the nitrogen 

 compounds is likewise saved, and it has been computed that beech 

 leaves send back into the stem 40% of their nitrogen compounds 

 during the yellowing period, while the chestnut may translocate 

 50% and the horse-chestnut as much as 65%. It should also be 

 added that many of the phosphorus and potassium salts are simi- 

 larly returned to the stem at this critical period in the life of the 

 leaf. Calcium salts, however, are not translocated to the same 

 extent but remain in the leaves and return to the soil. 



There is also a daily periodicity in translocation. During the 

 day the food is manufactured and accumulates in the leaf. At 

 night most of the products of the day's work are removed, and 

 the leaf starts in the following day "with a clean desk." There 

 is probably some removal during the day as well, but the maximum 

 movement occurs between sunset and midnight. Lack of starch 

 in a leaf does not indicate that translocation has occurred but 

 merely that the starch has been transformed (digested). That 

 such translocation does actually take place, however, can be easily 

 demonstrated by comparing the total carbohydrate loss from a 

 leaf attached to the plant with that from one which has been de- 

 tached. The loss in the former case has been found to be more 

 than five times that in the latter. 



Path of Translocation. — The path of translocation has been 

 a subject of much inquiry. In simple plants such as the algae, 

 floating water plants, etc., no special channels are necessary. 

 Diffusion from cell to cell, according to the ordinary laws of dif- 

 fusion (Chap. XVIII), is sufficient to account for all the move- 

 ment necessary. In woody perennials (trees and shrubs) and in 

 the larger herbaceous plants, it is at once evident that the ordinary 

 diffusion processes cannot proceed at a high enough rate to explain 

 the movement observed. 



Although Curtis (1920) has questioned the conclusions of pre- 



