AGRELIUS: FOOD-CONDUCTION IN PLANTS. 175 



It will be seen that the walls are commonly quite thin — ^those 

 between them and the companion cells being thinnest. Thick- 

 ness of their walls ranges from 0.25 micron to 4.17 microns. 

 Problem III. — The facts observed concerning the third part 

 of the problem are tabulated in table III. The tendency is 

 shown to be toward a reduction of the diameter of the vessels 

 in the fruitstalk and petioles, with an increase in thickness 

 of the walls. 



RESUME. 



First. — There are special devices for the better lateral trans- 

 fer of foods present in the phloem and cortex of the plants 

 studied. These are (a) pitted walls; (b) the arrangement of 

 the phloem in narrow wedges with pericycle cells sweeping 

 around these and connecting with the medullary rays, and 

 (c) radially elongated medullary-ray cells adapted readily to 

 conduct the food as stated. It is probable that in many plants 

 the extreme thinness and large area of the phloem walls per- 

 mits a considerable lateral movement of materials without 

 resort to pits, and that no pits are to be found in some (per- 

 haps many) cases. 



Second. — Sieve tubes with perforated sieve plates were dem- 

 onstrated in all but three of the plants examined, and here I 

 was satisfied of their presence. They often increase the area 

 of their sieve plates by placing them obliquely, and these may 

 require special thickenings to support them. The number of 

 the sieve tubes in various species varies considerably. They 

 are present in some plants when probably almost functionless. 



Third. — Relatively, the phloem and xylem do not vary 

 greatly in amount in the various parts of the plants, such as 

 the stems, petioles, etc., but the main differences in this respect 

 are in the other tissues. 



a-Univ. ScL BuU.. Vol. V, No. 10. 



