CHAPTER X. 



SECRETORY AND EXCRETORY SYSTEMS. 



I. GENERAL CONSIDERATIONS. 



All green terrestrial plants give off gaseous substances, in connection 

 with the functions of transpiration, photosynthesis, and respiration. In 

 the majority of cases various liquid compounds or, it may be, solutions 

 of solid bodies are also liberated through the mediation of secretory 

 organs composed of one or more specialised cells. The physiological 

 importance of this process of secretion is very diversified, just as it is 

 in the animal kingdom. Thus, many plants secrete liquid water, 

 when transpiration is suppressed, an arrangement which prevents the 

 hydrostatic pressure within the conducting system from becoming 

 excessive, and hence protects the ventilating system against the 

 danger of flooding. Such water - secreting organs, which vary 

 greatly as regards the details of their construction, are termed 

 hydathodes. Other plants, again, are furnished with special organs 

 for the secretion of digestive enzymes ; these structures, which may 

 be compared to the digestive glands of animals, obviously play an 

 important part in the general metabolism of the plant. Many secretions, 

 such as ethereal oils, resins, mucilage, etc., serve for various ecological 

 purposes, such as restriction of excessive transpiration, protection against 

 animal foes, or, on the other hand, as in the case of nectar attraction 

 of desirable insect-visitors. Finally, it is not improbable that some of 

 the secretory organs of plants are entirely concerned with excretion 

 of useless by-products of metabolism, like the kidneys or nephridia 

 of animals. 



Sometimes the products of the activity of secretory organs pass 

 out directly from the organism. In other cases they are, to begin with, 

 deposited within the confines of the plant-body, though outside the 

 secretory cells ; here exudation follows at a later stage, or may not 

 take place at all. The escape of the secretion from the plant-body 

 is therefore not an essential, though it is a common feature of vege- 

 table secretory organs. The real distinctive character, which at once 



