166 PART III. PHYSIOLOGY. [ 40 



rupted. The effect of this upon the tree is that the portion of 

 the trunk below the wound, and the roots, cease to grow, and 

 slowly die, whei'eas the trunk and branches above the wound 

 remain healthy and continue to grow until the roots are no longer 

 able to absorb water, etc., from the soil with sufficient activity. 

 Inasmuch as the cortical tissue, through which the sugar travels, 

 is necessarily also cut through, the operation deprives the lower 

 parts of the body of the whole of their supply of organic plastic 

 material from the leaves, but does not interfere with the conduc- 

 tion of water from the roots to the leaves. 



The sieve-tubes differ from the vessels of the xylem in that they 

 contain living protoplasm ; their function is therefore probably 

 not purely mechanical, but it is vital, though the relation of the 

 protoplasm to the conduction of proteids in the sieve-tubes is not 

 clear. 



The companion-cells, and in their absence the cells of the bast- 

 parenchyma, which abut on the sieve-txibes, apparently serve in 

 the leaves as the means by which the nitrogenous products of 

 anabolism are brought to the sieve-tubes, and in other parts as 

 the means by which the proteids of the sieve-tubes are distributed 

 to the adjacent tissues ; there is some evidence to show that these 

 cells themselves actually carry on the formation of the proteids 

 which form the characteristic contents of the sieve-tubes. 



/. The Glandular Tissue (p. 96). The essential function of the 

 glandular tissue is to secrete, and the secreta are either plastic 

 substances or waste-products. 



It may be stated generally that the excretion of plastic sub- 

 stances on the surface of plants has special reference to their 

 relation with insects. Thus, the excretion of sugar by floral 

 nectaries is to attract insects to visit the flowers, and thus to 

 ensure the advantages of cross-pollination at a certain, though 

 relatively inconsiderable, cost. The excretion of sugar by extra- 

 floral nectaries is an expense incurred by the plant with the 

 object of attracting to it insects of a kind which will keep off 

 noxious insects or other animals ; these organs are especially char- 

 acteristic of mymnecopMlous (ant-loving) plants, which by this 

 means provide themselves with a police of ants to keep off either 

 other injurious (e.g. leaf-cutting) species of ants, or insects of other 

 kinds (e.g. boring bees, etc.), or even herbivorous mammals. 



The secretion of waste-products has, as its immediate object, the 

 removal of these substances from the sphere of metabolism ; but 



