THE MATERIAL OUTGO OF PLANTS 



33,^ 



tissue and to deserve the name water gland; in others it seems to be 

 passive. 



Guttation in fungi. — Guttation is not confined to the higher plants, 

 nor are there always such elaborate accessory structures. It occurs in 

 its simplest form in many fungi. Thus Pilobolus crystallinus owes its 

 specific name to the droplets of water which appear on its sporangio- 

 phores (fig. 630), and Mendius lacrymans, the dry-rot 

 fungus, likewise, " weeps " so much water that it accum- 

 ulates in big drops on the surface of its sheetlike 

 mycelium. 



Nightly guttation. — In nature the checking of evapo- 

 ration, which results in guttation, occurs chiefly at night, 

 when many young plants exude water. What remains 

 adherent at the water pore may be partly resorbed when 

 transpiration begins. 



This seems to be the way in which a destructive bacterial dis- 

 ease of cabbage infects the plants. By contamination of the 

 hanging drop the bacteria find their way into the chamber as the 

 drop evaporates or is resorbed, there develop and so kill the 

 adjacent cells, whence they enter the xylem bundles and work 

 backward, killing and rotting the bundles. When the crop is 

 gathered and stored, they develop further, until the head is spoiled 

 by the extension of the blackened and rotted tracts in the blanched 

 leaves. 



Fig. 630. — 

 Sporangiophcre 

 of Pilobolus, 

 showing ex- 

 uded water. — 

 Adapted from 



ZOPF. 



One may easily observe the exudation of water from the leaves of 

 lawn grasses early in the evening, when the " dew " is said to be " fall- 

 ing." The warm soil conduces to the entry of water ; the cooler air 

 checks evaporation ; these conditions permit maximum turgor ; gutta- 

 tion at the tips of uninjured leaves, or, more often and more promptly, 

 bleeding from the cut ends of the leaves is the result. Dew, of course, 

 may form under proper conditions; but exuded water forms a great part 

 of what passes as dew. 



Artificial guttation. — Guttation may be produced artificially by injecting water 

 under pressure into the stem of a plant known to have water pores, as by attaching 

 the end of a cut shoot to a water tap. Presently droplets exude at the usual places. 

 It is usually assumed that the water is thereby forced through the plant tissues, 

 but as city water pressure varies from 2-3 atmospheres (seldom more, and less 

 will often answer), it is doubtful if so low a pressure (as compared with the 3-10 

 atmospheres of common turgor pressure) would be adequate to do this (see further, 

 P- 336). 



