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 haped 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 
