220 TRANSLOCATION IN PLANTS 



show no movement or only a greatly delayed movement 

 into them. This evidence supports the opinion held by 

 many that movement through the phloem of mature leaves 

 is restricted to export only. Phillis and Mason (1933), 

 however, observed a movement of sugar into darkened leaves 

 of cotton, and although this may have been restricted to 

 movement into conducting tissues only it does demonstrate 

 a reversal through the phloem of the petiole. Furthermore 

 the fact that white leaves, which are occasionally met with 

 on many kinds of plants, continue to live and receive foods 

 from the rest of the plant (Wee vers, 1923) offers conclusive 

 proof that some mature leaves can readily receive sugar 

 from the parent plant. That they can remain alive and 

 continue to receive food if darkened has not been tested 

 so far as I know. It would be of interest to determine 

 whether such white leaves receiving carbohydrates through 

 the phloem would at the same time export fluorescein or 

 viruses. If they would do so, as might be expected, it 

 would seem to demonstrate simultaneous transport in 

 both directions through the phloem. The continued export 

 of nitrogen (Schumacher, 1931), of fluorescein (Schumacher, 

 1933), and of viruses (Holmes, 1932; Caldwell, 1934) from 

 darkened leaves, at a time when sugar was probably moving 

 into the leaves, points rather strongly to a mechanism allow- 

 ing simultaneous movement in both directions. These 

 observations also indicate a mechanism that is active 

 independently of the presence or absence of material 

 to be transported. It is true, on the other hand, that in 

 spite of the evidence for movement of carbohydrate into 

 darkened cotton leaves, as well as into white leaves, we 

 have not conclusive evidence that darkened leaves which 

 are exporting virus or fluorescein are at the same time 

 receiving carbohydrate. As mentioned previously trans- 

 port in leaves that are approaching death or are about to 

 abscise may be strictly unidirectional and outward only. 

 Evidence on this point and even on the matter of normal 

 movements into and out of leaves is very meagre. 



39. The Necessity of Living Cells. — That living cells are 

 necessary for, and take an active part in, translocation 



