MECHANISM OF TRANSLOCATION OF SOLUTES 503 



oxygen is necessary for the sieve tubes to operate in translocation. Similarly 

 it has been shown that exposure of petioles to narcotics retards translocation 

 of foods out of the leaves. Demonstration that living cells are indispensable 

 for translocation does not necessarily prove, however, that their essential role 

 is that assumed by the streaming of protoplasm hypothesis. 



Like the mass flow theory the protoplasmic streaming hypothesis also has 

 some serious weaknesses. The more important of these will be summarized. 



1. Protoplasmic streaming has not been observed in mature sieve tubes 

 except in water plants, although it can be easily demonstrated in the phloem 

 parenchyma and companion cells. Young sieve tube elements also exhibit active 

 protoplasmic streaming, but all movement of the protoplasm appears to cease 

 as the sieve tubes mature. According to Crafts (1938) the protoplasm of 

 mature sieve tubes is in an inactive, highly permeable condition in which 

 streaming movements have neither been observed nor are to be expected. 



2. A second serious weakness in this theory is that the proposed mechanism 

 apparently will not account for calculated rates of translocation, at least as 

 it occurs in some plants. A number of estimates have been made of the rates 

 at w^hich solutes are translocated through the phloem. One of the most recent 

 is Crafts' (1933) calculation of the rate of movement of foods into a potato 

 tuber. All of the carbohydrates which enter a growing potato tuber pass 

 through the slender stolon connecting it with the parent plant. The tubers 

 on which this calculation was based were found to have increased in dry 

 weight at the rate of 0.89 g. per day. Measurements were also made of the 

 cross-sectional area of the various tissues of the stolon. As a result of these 

 measurements it was possible to compute that a lO per cent solution of sugar 

 must move at a rate of 19 cm. per hour if the total cross-sectional area of the 

 phloem, including the walls, serves as the channel of conduction, or at a rate 

 of 83 cm. an hour if the flow is restricted to the sieve tubes. Some of the 

 other similar calculations indicate even greater rates of movement than this. 



The bearing of these data upon the protoplasmic streaming theory is evi- 

 dent from further calculations by the same worker. A probable estimate indi- 

 cates that only about 5 per cent of the cross-sectional area of the phloem 

 could be occupied by protoplasm flowing in one direction. On the assumption 

 that protoplasm is carrying sugar in the pure state equal to its own volume 

 it must stream at a rate of 56.8 cm. per hour to account for the calculated 

 rate of transport. Since the proportion of sugar actually present in the sieve 

 tubes probably seldom exceeds 25 per cent it is evident that actual rates 

 of streaming must be several times as great as this. Furthermore, in order 

 to account for the observed rates of translocation through the relatively small 



