14 PASSAGE OF FOOD FROM LEAVES TO STEM. 
seen that diffusion takes place slowly in the cortex, and less rapidly 
than in any of the other tissues. When the colouring fluid is 
injected into the cortex alone it goes no further, only a few cells. 
surrounding the puncture are coloured, but that is all. This was 
markedly seen in the case of the box stem. The puncture was 
full of eosine, and yet not even the adjoining cells were coloured. 
There was a bright red residue on the stem, which indicated that 
probably some of the liquid had been absorbed, but the colouring 
matter left. 
When a cork cambium is present, the eosine enters its cells and 
diffuses from cell to cell. In cross sections the cork-forming 
layer appears as a red band. This was very noticeable in the 
section of the laurel stem. Longitudinal sections show that in 
the cortex the eosine diffuses as slowly in an up and down 
direction as in a lateral one. The case of Pinus sylvestris was 
very interesting, as in the cortical cells the eosine was found to 
have diffused more than in any of the other stems. A section cut 
two inches above the insertion of the eosine showed that it was 
especially the cells or epithelium surrounding the resin canals 
which were coloured. In most of the other stems the dye was 
found to have ascended farthest in the xylem vessels. 
In the phloem the eosine was found to have diffused to a 
greater extent in all directions than it did in the cortex. The 
distance ascended was in every case less than the distance 
descended. But until the experiment has been tried on growing 
plants at different seasons, and the results compared, no definite 
conclusions can be drawn. From the sections on the table you 
will see that it is possible to inject into the sieve tubes; and as the 
eosine when injected at one point only does not interfere with the 
life of the plant, both the rate and direction of the flow in°a 
growing plant might be accepted as being natural. Other dyes 
were tried, but eosine was found to be the best. 
That the course followed by the food material can be studied 
by the use of such a fluid seems probable, for besides the sieve 
tubes and companion cells, the medullary rays are also coloured. 
These latter extend from between the cells of the phloem across 
the cambium and xylem to the pith. The contents of the 
medullary ray cells diffuse into the cells of the cambium adjoining 
them. That the dye diffuses from cell to cell of the cambium in 
