NUTRITION 



ducting water pass through the entire body of the vas- 

 cular plant in the shape of long tubes, formed by the 

 combination of rows of cells; the cells themselves die 

 off, and their plasma content disappears. The stream 

 of water that rises constantly in these tubes is taken up 

 by the roots, conducted by the fibres to all parts, and 

 given off (transpiration) by the pores of the leaves. But 

 these pores also serve for the breathing of plants, being 

 connected with the air-containing intercellular passages ; 

 through these air-spaces, which serve for the aeration of 

 the higher plant-body, air and moisture can enter, and 

 oxygen be given off in respiration. Finally, many of 

 the vascular plants have special glands that serve for 

 secretion (of oil, resin, etc.). In the higher flowering 

 plants this division of work among the various digestive 

 organs gives rise to a very complicated apparatus for 

 nutrition. Among the many remarkable structures that 

 have been developed in this way by adaptation to special 

 conditions we may particularly note the organs for 

 catching and digesting insects in the insect-eating plants, > 

 the European drosera and utricalaria, and the tropical 

 nepenthas and dioncsa. 



The long scale of evolutionary forms which we find 

 in the tissue animals (metazoa) leads up uninterrupted- 

 ly from the simplest to the most elaborate physiological 

 functions and a corresponding morphological complex- 

 ity of organs. The two principal divisions of the meta- 

 zoa are chiefly distinguished by the circumstance that 

 in the coelenteria one single system of organs, the gastro- 

 canal system, discharges the whole (or most part) of the 

 partial functions of nutrition; while in the coelomaria 

 they are usually distributed among four different sys- 

 tems of organs, each of which is made up of a number of 

 organs. To an extent, we find once more in each great 

 division characteristic types of organization. How- 

 ever, comparative ontogeny teaches us that all these 



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