340 



PHYSIOLOGY 



The problem, therefore, is: How can solutes pass the ectoplast usually 

 impermeable to them ? The answer is merely in the form of a hypothe- 

 sis, like the one already proposed to account for 

 guttation and bleeding. If the accumulation of 

 the solute causes a rise of turgor, it is conceivable 

 that the very pressure itself might work such a 

 change in the cytoplasmic membranes that they 

 alter their permeability and permit the outrush 

 of water and its solutes in the direction of least 

 resistance, which will be toward the free surface. 

 Whether a renewed secretion will take place de- 

 pends on the further activity of the cell. Given 

 a repeated formation of the secretion, it might 

 escape again. The hypothesis then suggests a 

 rhythmic variation in the permeability of the cell 

 membranes, the secretion being formed inside the cell. 



Fig. 636. — Young 

 resin gland of fir {Abies) : 

 ti, duct, an intercellular 

 space formed by the sepa- 

 ration of the four nucleate 

 cells. — After Tschirch. 



This hypothesis is clearly inapplicable to secretions which are not miscible with 

 water, like essential oils and resins. They are probably formed, however, in the very 

 wall itself, and thus the material 

 may not have to traverse the ecto- 

 plast as resin or oil. Unfortunately, 

 even the place of their origin is still 

 obscure. 



R61e of certain secretions. — Nec- 

 tar is gathered by many insects, 

 some of which store it, after partial 

 digestion, as honey. While the 

 floral glands are being explored for 

 nectar, the visitors become dusted 

 with pollen and transfer this to ripe 

 stigmas of the same or other flowers, 

 thus insuring pollination in many 

 cases where otherwise it might not 

 occur (see Part III on pollination). 

 The role of extrafloral nectar is not 

 clear. Digestive glands, most defi- 

 nite in insectivorous plants (p. 386), secrete enzymes (p. 399) by which the soft parl< 

 of captured insects are dissolved. Essential oils (p. 413) sometimes prevent plants 

 from being eaten by animals. 



Fig. 637. — Oil receptacle (a) in orange (Cit- 

 rus Aurantium), formed partly by splitting, but 

 chiefly by destruction of secreting cells and their 

 neighbors (0 ; 0, 0^ drops of essential oil. — 

 After Tschirch. 



