188 



BOTANY 



PART I 



have shown that a finely perforated septum, if the perforations are a 



certain distance apart, offers no obstacle to diffusion. C) 



Movements of diffusion may also take place Avithin a cell if dissolved 



substances are not at the same concentration throughout the cell. 



Movements of diffusion proceed quite slowlj^ The rapidity with 



which mixing occurs may 

 be greatly hastened if a 

 movement in mass be added 

 to that due to diffusion. 

 In common life and in the 

 laboratory this is effected 

 by shaking the solution, 

 and within a cell the same 

 result may be obtained, e.g. 

 by the protoplasmic move- 

 ments. The greater the 

 length of a cell the more 

 suitable is it for conductinn; 

 material through the plant, 

 since the slow diffusion- 

 movement need only take 



l^lace at long intervals, i.e. 

 at the ends of the cell ; in 

 the intermediate portion 

 movements of mixing play 

 a large part. The longest 

 cells, such as the laticifer- 

 ous tubes lead on to the 

 elongated spaces in the 

 body of the plant, formed 

 by vessels and by inter- 

 cellular spaces. 



Water can move in a 



closed stream within the 



which may be 



capillary tubes a metre in 



Fid. 178. — Experiment to show the direct ooiniiuuiication 

 of the external atniosiiliere with the internal tissues of 

 plants. The glass tube R, and the leaf P, are fitted air- 

 tight in the bottle G ; upon withdrawal of the air in tlie 

 bottle by suction on the tube 11, the external air pene- 

 trates the intercellular spaces of the leaf through the 

 stoniata and escapes in the form of small air-bubbles VCSSels, 

 from the cut surface of the leaf-petiole. (From Detmer's 



length, just as it does in 

 a glass tube. That this should take place requires either a pressure 

 from below or a pull from above. A well-known physical law teaches 

 us that the amount of water passing through a capillary tube in unit 

 time and with a given pressure is proportional to the fourth power 

 of the radius of the tube, and inversely proportional to its length. 



In the intercellular spaces gases are as a rule found, but 

 sometimes they contain water and not infrequently secretions. 

 The movements of gases may either be brought about by movements 

 of diffusion due to differences in gaseous contents of the intercellular 



