

ORGANIZATION OF HIGHER 

 PLANTS; THE TRANSPORT OF SAP 



(Readings: Weisz, pp. 171-199; 231-239. S. P. T., pp. 55-63; 137-141; 368-378. 

 Villee, pp. 104-106; 117-129. Review the discussion of the plant kingdom in 

 Exercises XII and XIII. V. Grant, "The Fertilization of Flowers," Sci. Am. 

 184, No. 6, 52-56, June 1951, Reprint No. 12. M. H. Zimmerman, "The Move- 

 ment of Organic Substances in Trees," Science, 133, Jan. 13, 1961, pp. 73-79.) 



This week we shall examine the organization 

 of flowering plants (angiosperms), which repre- 

 sent the peak of plant evolution, just as the 

 vertebrates studied last week represent the peak 

 of animal evolution. We shall also inquire into 

 an important aspect of their function, the trans- 

 port of sap, which plays a role in vascular plants 

 comparable with the circulation of blood in ani- 

 mals; and into the osmotic relations of plant 

 cells, upon which the transport of sap largely 

 depends. 



Surely you are already familiar with the gross 

 division of higher plants into stems, roots, leaves, 

 and flowers. In the course of this period we shall 

 examine these organs more closely, dissect a 

 flower and a fruit, and examine under the micro- 

 scope the tissues of which such organs are com- 

 posed. 



We shall begin, however, by setting up experi- 

 ments on the rise of sap, and on plasmolysis. 

 Once these have been started, they need only 

 occasional attention; and while they are going 

 on, you can examine the anatomy of plants and 

 plant structures. 



WATER MOVEMENT IN PLANTS 



One of the major problems in the life of vas- 

 cular plants is the transport of sap. This flows 

 in two streams, one generally downward, carry- 

 ing organic molecules prepared by photo- 

 synthesis in the leaves; the other upward, carry- 

 ing water and dissolved ions absorbed from the 

 soil by the roots. For the plant to survive and 

 grow, both streams must penetrate to all its 

 tissues. 



Of these two streams, the upward stream of 

 water and salts from the roots is the larger and 

 more continuous. A fraction of it supplies the 

 downward stream, and to this degree we may 

 speak of the flow of sap as a "circulation." A 

 further fraction contributes to the growth of the 

 plant, and is retained in new tissues. Much of the 

 ascending water, however, is lost by evaporation 

 from the leaves. 



The upward stream presents the major prob- 

 lem. To bring sap from the roots to the top of a 

 high tree demands a very large force. The high- 

 est trees — California redwood, for example, and 



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