140 TEXTBOOK OF PLANT PHYSIOLOGY 



uninterrupted supply of nutrient substances. If this supply is 

 checked, as may be observed in the usual experiments with the 

 guttation of plants, when the aerial organs are cut, bleeding rapidly 

 decreases, or ceases. 



To understand the cause of this phenomenon, the mechanism 

 of root pressure will be examined. It has been seen that the water 

 enters the root by virtue of its osmotic suction, and, having passed 

 through a series of parenchyma cells it is driven with considerable 

 force into the vessels. From the osmotic system of the cell, how- 

 ever, no conclusion can be drawn as to the reasons for the possible 

 expulsion of water from the cell. Although Dutrochet, who was 

 the first to investigate osmotic phenomena in plants, compared 

 the rise of the sap in the vessels of roots and 

 stems to the rise of the liquid in the tube of an 

 osmometer, the similarity is not very great. 

 The tube of the osmometer is the direct con- 

 tinuation of its cavity. The rise of the liquid, 

 therefore, is simply the result of the general 

 increase in volume of the same uniform solution 

 filling the osmometer. In the root, on the con- 

 trary, the cavities of the vessels are separated 

 from the adjoining cells through which the 

 Fig. 56.— Guttation water is driven. Besides, it is not the cell sap 

 of.thesporangiophore j-,^ a considerably more dilute solution, that 



of Pilobolus {adapted , T , 



from Lepeschkin). ascends through the vessels. In order to visu- 

 alize root pressure, it must be accepted that at 

 least some root cells are able simultaneously to imbibe water and 

 to expel it into the vessels. Therefore, the osmotic mechanism 

 of the cell already discussed, must be somewhat modified in 

 order to understand this flow of water in one direction. 



For this purpose several modified mechanisms have been sug- 

 gested. At present, it cannot be said with certainty which of these 

 is correct. The theory proposed by Lepeschkin seems to have 

 gained the widest recognition. This theory is based on the suppo- 

 sition that an unequal permeability of the protoplasm exists in dif- 

 ferent parts of the cell. Lepeschkin was led to this idea by his 

 observations on the water drops exuding on the surface of the 

 sporangiophores of a fungus, Pilobolus (Fig. 56). It is well known 

 that these fungi possess no septa within their hyphae. From an 

 osmotic point of view, therefore, their whole body represents, as it 



