AGRELIUS: POOD-CONDUCTION IN PLANTS. 173 



of these pits and their contact with the companion cells for 

 taking up and transferring the food materials laterally, espe- 

 cially to the cambium. Figure 6 shows a very common ar- 

 rangement of the medullary rays which bears out the rest of 

 his statement. It may be added to by saying that the medul- 

 lary rays are often close to each other tangentially, and the 

 rays and ray-like cells of the pericycle often sweep around the 

 narrow portions of the phloem, as seen in figure 6 of Amoy^pha 

 fi-uticosa, as though for the special purpose stated above, and 

 in a way especially fitting them for that function. Some re- 

 cent research^ has further verified Strasburger's statements 

 in certain plants.^ 



One thing to be noted in botanical literature is that work on 

 sieve tubes and sieve plates is usually done with plants from 

 the Vitacese, Cucurbitacese, Tiliacese, and a few others, where, 

 as will be seen by reference to table I, the walls of the phloem 

 are unusually thick. Presumably this is because here the 

 things to be seen are more pronounced, and hence more easily 

 found and studied. Table II shows comparative data collected 

 from the plants having pits in the lateral walls of the phloem 

 as observed in this work and from those in which such pits 

 were not observed. In Equisetum pits are general in the 

 phloem elements and in the rest of the cortical tissues. In 

 others this is not true. In the tables mentioned it will be seen 

 that pits are present in those plants having thicker walls than 

 those in which pits were not observed, and, as a rule, the sec- 

 tions were from actively functioning tissue. It is seen that 

 the phloem walls are as thin in some cases as 0.25 micron, and 

 in many cases 0,35 micron. In fact, in the phloem in many 

 instances there was but little evidence of appreciable addition 

 in thickness to the original vertical wall formed by the cam- 

 bium or procambium. As a result of this, in drawing with the 

 high magnification of 1200 diameters, it was not feasible to 

 use a double line for representing the thickness of the walls, 

 and the attempt to do so was soon abandoned and choice made 

 of the measurement and tabulation method as shown by the 

 tables. It is quite plain that such thin walls could neither 

 have nor require pits. In the xylem I found pits invariably 



7. Hill, A. W., Histology of the Sieve Tubes of Angiosperms. Annals of Botany, 

 vol. 22, No. 86, pp. 246-290. 



8. Sykes, M. G., Anatomy and Histology of Macrocystis pyrifera and Laminaria sac- 

 charina. Annals of Botany, vol. 22, No. 86, pp. 291-326. 



