694 ECOLOGY 



phenomenon especially common in water plants, sometimes may facilitate 

 conduction in living cells. 



The rdle of sieve tubes. — The sieve tubes generally are regarded as 

 organs of protein conduction, partly because they are living elements 

 rich in protein, partly because they form a continuous system of tubes 

 comparable to the water-conducting elements, partly because the porous 

 sieve plates seem well fitted for the passage of viscous albuminous ma- 

 terials, and partly because no other tissues are known that are peculiarly 

 fitted for protein conduction. The girdling of trees often results in 

 increased growth above the girdled area (fig. 665), and under favorable 

 conditions roots originate from that region. Such phenomena appear 

 to indicate that there is in the bark a downward movement of foods and 

 that they tend to accumulate above the girdled portion, furnishing the 

 materials used in the extra growth. Trees often are killed by complete 

 girdling, probably because of the inability of the food to reach the roots. 

 In those dicotyls which have sieve tubes inside the hadrome, none of 

 the above-noted effects of girdling are seen. 



Sieve tubes have been variously regarded as organs of protein accumulation, as 

 organs of protein manufacture, and as structures that are well fitted for the propa- . 

 gation of stimuli. The conduction theory, however, seems most tenable, though 

 the actual movement of material has not been well observed. Movement is from 

 regions of high to regions of low pressure and may be downward toward the roots, 

 outward toward the branches, or upward toward the flowers and fruits. Owing to 

 the chemical similarity between the protoplasm and the conducted protein, it may 

 be regarded as advantageous or even necessary that the conducting cells be living. 

 The closing of the sieve plate in winter by a callus may be advantageous, but more 

 probably it is of no particular significance. Nothing is known concerning the r6le 

 of the companion cells, which in angiosperms persistently accompany the sieve 

 tubes and even extend beyond them in the bundle termini. 



Conduction in the lower plants. — The sieve tubes of the larger algae commonly 

 are supposed to be organs of protein conduction, but this has not been proved. 

 The rise of colored fluids has been observed in the " vascular " tract of moss 

 stems ; it is not known that movement is more rapid there than in the other stem 

 tissues, though the greater cell elongation makes it seem probable. 



The path of carbohydrate conduction. — Sugar passes readily from 

 cell to cell by osmosis, and it is probable that the cortical parenchyma 

 is the chief tissue involved in its conduction, though carbohydrate as 

 well as protein may pass along the sieve tubes ; the ill effects of girdling 

 may be due as much or more to the cutting off from the roots of carbo- 

 hydrates as to the cutting off of proteins. The endodermis, because of 

 its rich starch content, has been regarded as a region of carbohydrate 



