CHAPTER I. GENERAL PHYSIOLOGY. 679 



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 (p. 180), which abut on the sieve- tubes, 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. 



In certain cases (e.rj. Mimosa pudicd) some of the parenchymatous cells of the 

 bast are specially modified apparently for the conduction of stimuli. 



/. The Glandular Tissue. 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 thns to 

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

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

 floral nectaries (p. 138) 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 

 characteristic of myrmecophilous (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. 

 Perhaps the most remarkable instances of this kind is the pro- 

 duction of small, solid, cellular " food-bodies " on the tips of the 

 leaf-pinnules of a myrmecophilous species of Acacia (A. sphceru- 

 cepliala). 



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

 the removal of these substances from the sphere of metabolism ; 

 but their deposit at or near the surface serves the purpose of 

 protection in various ways. For instance, the secretion of wax on 

 the surface is an obvious protection against wet. Similarly there 

 can be little doubt that when the system of resin-ducts, in plants 



