ILLUSTRATIVE STUDIES 173 



diffusion is a slow process and it alone cannot suffice to cany 

 foods fast enough when growth is rapid or when at the height 

 of photosynthesis the leaves are taxing the utmost capacity 

 of the conductive tissues. To illustrate the slowness of move- 

 ment by diffusion alone: common salt in comparison with many 

 other substances diffuses rapidly, yet in a lo per cent, solution 

 it required in one experiment 319 days to transfer a milligram 

 of salt one meter, and under like conditions it would take fourteen 

 years for egg albumin to travel the same distance. So we must 

 assume that in plants diffusion is assisted in various ways, such 

 as jarring due to the wind, etc., changes in temperature, circula- 

 tion of the cytoplasm, and possibly in other ways. We know 

 that the contents of the sieve tubes are under pressure, for the 

 tubes empty themselves when cut open, and this pressure would 

 propel materials toward the places where for immediate use or 

 for storage they are being removed from the tubes by the sur- 

 rounding tissues. It is furthermore quite possible that effective 

 conditions and forces are present of which as yet we have no clew. 



Illustrative Studies 



1. Study the phloem elements in the stem of squash, grape, 

 and hop. Mount free-hand cross and longitudinal sections in 

 dilute glycerine. Hunt for sieve tubes, companion cells, and 

 phloem parenchyma. Find the perforations in the sieve plates 

 in both cross and longitudinal sections. How far apart are 

 the plates? and how many of them would food have to pass 

 through in going i cm. ? Draw sieve tubes from both points 

 of view, showing the sieve plates. 



2. Study border parenchyma (page 146) and phloem ele- 

 ments in cross and tangential sections of leaves. How far 

 does the food have to travel from the palisade and spongy 

 parenchyma before it can reach the conducting cells of the veins ? 

 Make drawings that will best illustrate what you have seen. 



Sections made from material imbedded in paraffin are best 

 for this study, but good free-hand sections will do. To cut 



