396 PHYSIOLOGY 



Rhythmic iranslocation. Since leaves are the principal regions of 

 food making, which is distinctly rhythmic by reason of the alternation 

 of light and darkness, the translocation of food shows a corresponding 

 rhythm. The transfer of any soluble food is continuous, and the rate 

 is determined by the usual factors ; but, as the transportation facilities 

 are overtaxed during the day, there is on the whole an accumulation of 

 food in the leaves then; only after the nightly slackening does emptying 

 of the leaf become obvious. 



That a leaf which shows starch near the close of a day may show none in the 

 early morning does not necessarily indicate that carbohydrates have been carried 

 off during the night, though they doubtless are, but only that they have been re- 

 duced in amount in some way, probably by migration and by conversion into other 

 foods. 



Causes of movement. Nothing is satisfactorily known as to the 

 causes of movement in the phloem. In the sieve tubes the absence of 

 protoplasmic membranes closing the ends surely permits more rapid 

 diffusion, which may be further facilitated by mechanical mixing due to 

 bending and other compression of parts of the system. That the con- 

 tents are under pressure is shown by the rapid oozing of material from cut 

 sieve tubes, an amount being reported in Cucurbita which indicates that 

 one or even two internodes had been emptied, and so the material must 

 have passed 75 to 100 of the sieve plates (the perforate end walls of the 

 sieve cells). The source of this pressure and the effect of it on translo- 

 cation is not known. 



Latex system. In certain families, 1 it may be that translocation of 

 foods takes place through the latex vessels, as well as by the phloem. 

 Latex vessels form a system of branched or anastomosing tubes run- 

 ning through the cortex (more rarely elsewhere), and ending blindly 

 in the leaves and roots. Histologically, they are coenocytes or cell 

 fusions (see Part I, p. 27). They approach very near to the growing 

 points, and in the leaves have close relations with the manufacturing 

 cells, the very arrangement sometimes suggesting its fitness for collect- 

 ing, foods. The latex which fills these tubes is the cell sap of a huge 

 vacuole, the protoplasmic contents being reduced to a very thin layer. 

 Latex is in part a watery solution of many substances, such as proteins, 

 sugars, gums, tannins, alkaloids, and salts; in part an emulsion of oils 

 and tannins in droplets; and in part suspended granules of starch, gum, 



1 Particularly the Papaveraceae, Compositae (Cichorieae), Lobeliaceae, Campanu- 

 laceae, Asdepiadaceae, Apocynaceae, Euphwbiaceae, Moraceae, Araccae, and Musaceae, 





