196 BOTANY I-AKT i 



Seeds and spores, after separation from their parent plants, remain productive for 

 a long time ; seeds of Nelumbium, which had been kept dry for over one hundred 

 years, proved capable of germination. A similar vitality was shown by moss 

 spores which had lain in a herbarium fifty years. The often-repeated assertion 

 concerning the germination of wheat found with Egyptian mummies ("mummy- 

 wheat") has, however, been shown to be erroneous. Many seeds lose their power 

 of germination after having been kept dry for only a year ; others, even after a few 

 days ; and others again cannot endure drying at all. On the other hand, the 

 seeds of some water plants (Eichhornia, etc.) germinate better after being dried 

 for a period. It must not be forgotten that in all these instances a certain 

 amount (about 9-14 per cent) of hygroscopic water is retained by plants even when 

 the air is quite dry. Over the sulphuric acid of the desiccator, seeds retain for 

 weeks 6 per cent or more of their weight of water. Even drying at 110 or the 

 action of absolute alcohol can be borne by some spores and seeds ( 21 ). 



Apart from permeating and energising the cells, water has other 

 and more varied uses in plant life. It is not only indirectly indis- 

 pensable for the solution and transport of the products of metabolism, 

 but also directly in that its elements, hydrogen and oxygen, are made 

 use of in the formation of organic compounds in plant nutrition. 

 Water thus used (cf. p. 219) may be designated CONSTITUTION WATER ; 

 for example, in the formation of every 100 grammes of starch or 

 cellulose 55 grammes of constitution water, and in the formation of 

 the same amount of glucose 60 grammes of water, are used. Water 

 is also necessary for the turgidity and consequent rigidity of 

 parenchymatous cells (p. 179); it is of use in the process of the 

 growth of plant cells, which take it up in large quantities, and, 

 through their consequent expansion, enlarge their volume with but 

 little expenditure of organic substance. 



A further and still more important service which water performs 



for plants consists in THE CONVEYANCE AND INTRODUCTION INTO THE 

 PLANT BODY OF THE NUTRIENT SUBSTANCES OF THE SOIL. Although 



a large amount of water is retained in the plant body (up to 96 per 

 cent in succulent tissiies) for the maintenance of rigidity and enlarge- 

 ment of the organs, a still larger quantity of the water taken up by 

 the roots passes through the plant merely as a medium for the trans- 

 port of nourishment, and is again discharged through the leaves by 

 evaporation. By this TRANSPIRATION from the aerial part of plants, 

 the water passing into them from the roots escapes, and at the same 

 time, by preventing saturation, which would otherwise be produced, 

 a continuous upward movement of the water is maintained. The 

 current of water thus produced is accordingly termed the TRANSI-IKA- 

 TION CURRENT. As the result of evaporation only water, in the form 

 of vapour, and gases can escape from the plant. As THE WATERY 



FLUID ABSORBED BY THE ROOTS CONTAINS SALTS, AND OTHER NON- 

 VOLATILE SUBSTANCES IN SOLUTION, THESE ON EVAPORATION ARE 

 LEFT IN THE PLANT AND GRADUALLY INCREASE IN QUANTITY. This 



accumulation of mineral salts is absolutely necessary for the plant, 



