Buds and Stems. 59 



downward. The sieve tubes in the phloem portion of the 

 bundle outside the cambium ring (see Fig. 21) seem to be 

 .the highways for these food materials. The sieve tubes 

 are 'thin-walled and allow the food materials to pass to and 

 from the surrounding tissues as needed. They have cross 

 partitions at frequent intervals to strengthen them, but the 

 partitions are perforated so that materials may pass rapidly 

 through them. 



41. Effect of Girdling. When a tree is girdled, the sieve 

 tubes are removed with the bark (see Fig. 21), and the parts 

 of the stem and of the roots below the girdle are no longer 

 supplied with food. While the portions of the stem above 

 the girdle increase in thickness and length as usual, all 

 parts of the plant below the girdle are restricted in growth 

 because the main food supply is shut off. If the bark of 

 a stem is removed early in the spring, before the leaves are 

 out, the usual process of the unfolding of leaf and flower 

 buds may still take place because the reserve materials 

 stored in the underground parts and in the lower portion of 

 the trunk may be carried upward by the water currents in 

 the tracheal tubes ; but the following year, if the bark has 

 been completely removed in the girdle, and no twigs, bear- 

 ing leaves have been allowed to grow below the girdle, the 

 roots will not have stored in them the food necessary to 

 the production of new rootlets and root hairs, and thus 

 can no longer absorb the water necessary to the resump- 

 tion of growth. The advantage of girdling trees which 

 are to be cut, down lies in the fact that the stump and 

 roots below tHe girdle will thereby be prevented from re- 

 ceiving the food which might be employed in the produc- 

 tion of new shoots from adventitious buds on the stump or 

 roots. In stems of the type of Indian corn and palm there 

 can be no separation of the food-conducting and the water- 



