206 PHYSIOLOGY OF NUTRITION. 



to root pressure and related phenomena are much more compli- 

 cated than those above indicated, and it is especially to be em- 

 phasised, that in the plant cell the protoplasm (and particularly 

 its ectoplasm) plays an important part when pressures are set up 

 in plants by processes of osmosis ; but nevertheless the above 

 experiment is of considerable interest in connection with plant 

 physiology. In the cells of the parenchyma of roots, pressures 

 are set up owing to the osmotic capacity of the cell-contents. 

 The strongly turgescing cells finally press out a part of their 

 contents, which passes into the elements of the wood, and is driven 

 up within them. 



80, Further Experiments on the Escape of Liquid Water 

 from Plants. 



When, owing to osmosis, intense pressures are set up in the cells, 

 which finally overcome the resistance to filtration offered by the 

 hyaloplasm and cell- walls, quantities of fluid are forced out of the 

 cells. The experiments respecting the flow of fluid from the de- 

 capitated plants owing to root pressure have already familiarised 

 ns with phenomena whose cause is to be sought in osmotic pres- 

 sures, but there still remain a series of other phenomena which 

 must here be considered. 



Young pot plants of maize or Tropeeolum are placed in a ther- 

 mostat (see Fig. 76), and covered with a bell-glass. The soil in 

 which the plants are rooted has previously been well watered, 

 and the temperature in the apparatus is to be kept constant at 

 20-25 C. The transpiration of the plants under these conditions 

 is almost nil. Owing to the root pressure, all the cavities within 

 the plant become filled with water, and water may even escape to 

 the outside. We observe in fact that in the course of from half 

 an hour to two hours, drops of water appear on the tips of the 

 leaves in Zea, and on the edges of the leaf in Tropseolum, which 

 on reaching a certain size fall off and are replaced by new ones. 

 In Zea the water passes out through rifts in the epidermis, in 

 Tropeeolum through water stomata. 



That pressures as set up, e.g., by root pressure, play an important 

 part in causing the escape of drops of water from leaves is easily 

 demonstrated. To the shorter limb of a glass tube filled with 

 water, we connect a cut shoot of Vitis, Tropa?olum, or Impatiens 



