352 



PHYSIOLOGY 



(a, figs. 645-647), which contain water. As the water evaporates it pulls the cell 

 walls together, and in doing so straightens the ring and tears open the weak side. 

 The thick elastic C" S ^H walls of the cells resist this compression, until finally 

 the cohesion of water in the wall with the free water in the lumen is overcome, and 

 the sudden elastic recoil of the annulus hurls the spores as from a sling. 



Figs. 645-647. — Rupture of sporangium of a fern (Polystichum acrostichoides): 645, 

 the sporangium cracked; a, the annulus; 646, position of complete reversion, many of 

 the spores adherent to the upper part of the sporangium; 647, position after recoil, the 

 sporangium emptied; dotted lines in this figure show the position as in 646. — After 

 Atkinson. 



This cohesion is predicated of the columns of water which occupy 

 the tracheids and tracheae of the xylem, and it is coherent even through 

 the end and side partitions (see theory of relation of water and cell wall, 

 p. 301). If now any adequate lifting force could be applied at the upper 

 end, the cohesion of the water is sufficient to enable it to hold together 

 even to the roots of the tallest trees. That lifting force is evaporation, 

 and the osmotic relations of water in the live cells of the leaf furnish the 

 connection. Why the water columns do not break wherever bubbles 

 of gas appear (and they must appear whenever the column is under any 

 considerable strain), is not satisfactorily explained; and other like 

 difficulties appear. Yet this theory at least faces in the right direction, 

 seeking to give an account of the rise of water in purely physical terms. 

 However, as this phenomenon has baffled investigators for more than a 

 century, it may be a long while before it can be satisfactorily described. 



4. OTHER LOSSES 



Gases from the shoot. — Quite apart from the liquids and water vapor 

 which escape from the aerial parts, there are gases which are constantly 

 set free and leave the plant as such. These are carbon dioxid and 

 oxygen; the former is one of the usual end products of respiration, and 



