504 



LATERAL TRANSLOCATION OF SOLUTES 



apertures of the sieve pores sugar must move in a pure state at a rate of 

 270 cm. per hour. 



The maximum rate of streaming observed by Crafts in the phloem 

 parenchyma of the same species was 1.8 cm. per hour. Protoplasmic stream- 

 ing at a rate of 47 cm. per hour has been observed in cells of water plants 

 under favorable conditions, and it seems entirely possible that cyclosis in the 

 sieve tubes might occur at rates considerably in excess of the value given 

 above for parenchyma cells. Nevertheless, the rate of streaming seemingly 

 required to account for the measured rates of transport would be considerably 

 greater than this, and it seems improbable that cyclosis 

 occurs at such rates in the sieve tubes. 



Lateral Translocation of Solutes. — As shown in 

 Chap. XV lateral translocation of water in a tangential 

 direction readily occurs in woody stems. This does not 

 appear to be true of many solutes. In straight-grained 

 trees the sugars from the leaves on one side of a tree are 

 translocated chiefly to the roots directly below them. Simi- 

 larly if nitrates are added to the soil on one side of a tree, 

 nitrogen is transported principally to the lea\'es and branches 

 directly above the roots on that side (Auchter, 1923). 

 Lateral transport of at least some kinds of solutes in a 

 tangential direction therefore appears to be slow or negli- 

 gible in intact stems. Radial transport of solutes along the 

 vascular rays probably occurs much more readily than their 

 lateral movement in a tangential direction. 



Experiments of MacDaniels and Curtis (1930) are 

 also of significance in connection with the problem of lat- 

 eral translocation. When the trunks of apple trees were 

 spirally ringed (Fig. 113) and sodium nitrate added to the 

 gram to show spiral .^ ^^^ ^^^^^^ ^^^^ nitrogen was transported principally 



ringing or a woody ' , r 1 • 1 / n\ u-i 



to the branches above the open end of the spn'al [B), while 



those above A received relatively little nitrogen. In other 

 rds the nitrogen passed spirally around the trunk until it reached the end 

 of the ring, from which point it was transported vertically up the stem. The 

 same results were obtained whether only the phloem was removed, or whether 

 the phloem plus the outermost ring of xylem was cut out. This indicates 

 that the nitrogen was actually being transported through the phloem since 

 cutting this tissue had as much effect as cutting both the phloem and xylem. 

 Since amino acid synthesis in the apple occurs preponderantly in the roots 



Fig. 113. 



Dia- 



stem. 



wo 



