. A textbook of botany for colleges and universities ... Botany. 352 PHYSIOLOGY (a, 6gs. 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 Cl-shaped 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 Figs. 645-647. — Rupture of sporangium of a fern (Polystichum acrostichoidcs): 645, the sporangium cracked; o
. A textbook of botany for colleges and universities ... Botany. 352 PHYSIOLOGY (a, 6gs. 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 Cl-shaped 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 Figs. 645-647. — Rupture of sporangium of a fern (Polystichum acrostichoidcs): 645, the sporangium cracked; o, 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 appUed 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 wh
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