CHAPTER X 



TRANSPORT OF FOODS THROUGHOUT THE PLANT 



Need of Transporting Tissues. — As we have seen in Chap- 

 ter IX, most of the food is manufactured in the leaves and must 

 be carried from them throughout the plant wherever it is needed 

 for supplying materials and energy for growth and repair or 

 other purposes, or where food is to be stored up for future use. 

 The higher plants, namely, the Vascular Cryptogams, Gymno- 

 sperms and Angiosperms, have attained to such size that the 

 distance to be traveled by the food is often great; and it has been 

 found that short cells such as occur in the meristematic cells 

 of growing points, or in the pith or outer bark of older parts will 

 not suffice for carrying food except through very short distances. 

 The sieve tubes and associated cells of the phloem alone are 

 able to do this. Without them the conduction of food could 

 not take place any more than water could be carried in sufficient 

 amounts without tracheal tubes and tracheids. 



In following the phylogeny of tissues in the lower plants we 

 find that the evolution of the food-conducting, as well as water- 

 conducting, tissues is clearly in correlation with the evolution 

 of leaves, which by their efficiency in food-making, and their 

 large transpiration surfaces, have created a demand for means 

 of conducting both food and water more rapidly than can be 

 done by short unperforated parenchyma cells. In the mosses, 

 represented by Polytrichum commune, for example; these con- 

 ducting tissues have made a distinct beginning. Here the center 

 of the stem is occupied by a vascular bundle of the concentric 

 type (see page 43) with the water-conducting surrounded by 

 the food-conducting tissues. 



The development of the phloem from the procambium has 

 been told in Chapter II; but it will be useful here to review the 



157 



