THE MOLECULAR FORCES IN PLANTS. 233 



86. The Rate at which Water Moves in the Plant. 



It lias often been attempted to form an idea of the rapidity 

 with which water moves in the plant by placing the objects of 

 investigation with their bases in a solution of colouring matter, 

 and determining the height at which the colouring matter can be 

 detected after a certain time. This method, however, cannot lead 

 to accurate results. The absorbed pigment solution, viz., disin- 

 tegrates. The colouring matter is arrested by certain elements of 

 the tissue (especially the lignified ones), while the water travels 

 on. We can easily convince ourselves that such an analysis of 

 the pigment solutions is possible by pouring into a tall glass 

 cylinder an aqueous solution of methyl green or eosin (the latter 

 of such strength that a layer 10 cm. in thickness is still trans- 

 parent), and covering with a glass plate to which a narrow strip 

 of blotting-paper has been attached so as just to dip at the bottom 

 into the pigment solution. After a short time the colouring 

 matter will have risen to a certain height in the paper ; but the 

 paper is moist beyond the limit to which the pigment has pene- 

 trated. The water has thus risen higher than the pigment. 



If, on the other hand, we allow a strip of blotting-paper to dip 

 into a solution of Lithium nitrate of about 2 p.c. strength, it is 

 easy to prove that the Lithium salt rises to the same height as the 

 water. Thus, if we cut off the uppermost portion of the paper 

 reached by the fluid, and hold it in the flame of a Bunsen burner, 

 the presence of Lithium can be readily observed spectroscopically 

 (by the presence of the well-known red Lithium line in the 

 spectrum). The solution of Lithium nitrate has often been used 

 by Sachs 1 for determining the rate of movement of water in 

 plants, and we will make our experiments according to his method. 



It is best at first, for reasons which are not far to seek, to 

 experiment with perfectly intact rooted plants, and not with por- 

 tions severed from the parent. We may, e.g., employ willows. 

 Shoots of last year are cut in spring, and placed in food solutions. 

 In a few months, when the shoots have developed a copious root 

 system and numerous leaves, they are ready for investigation. 

 Maize plants raised by the water-culture method may also be em- 

 ployed ; and so also may pot plants of Nicotiana, Cucurbita, and 

 Helianthus, etc., grown in good garden soil. The plants must be 

 vigorous and very leafy. One or two days before the investigation 

 proper begins, we place them in front of a window with a south 



