THE FOOD OF PLANTS. 57 



by a cut running alongside the midrib. The halves provided with 

 midrib are at once placed in a moist atmosphere under a bell- 

 glass, in order to keep them turgescent ; the cut surface of the 

 leaf-stalk dips into water. The other halves are placed in a 

 poorly lighted part of the room till they have become somewhat 

 limp. Both sets are now placed in a ventilated glass receptacle, 

 the former with their stalks still in water, and exposed to direct 

 sunlight. In order to prevent undue warming, we interpose in 

 the path of the sun's rays a parallel- walled glass vessel containing 

 water. After they have been isolated for about three hours, the 

 Iodine test is applied to the objects. The limp halves have formed 

 no starch, the others kept fresh have formed large quantities of it. 

 The withering in this case and the like is to be observed in very 

 many plants has caused the stomata to close, and consequently 

 the admission of Carbon dioxide and activity of assimilation has 

 practically been brought to an end. If we experiment with 

 halved leaves of Hydrangea hortensis we find that even the limp 

 halves, if not too far gone, also produce starch in the sunlig'ht. In 

 this plant namely the stomata do not close when the tissue withers. 



We grow seedlings of Zea Mais, some in normal food solution, 

 others in a food solution to which we have added 0'5 per cent, of 

 common salt. The former thrive well, while the latter, although 

 their tissues do not undergo any striking changes, are completely 

 retarded in their development. In the leaves of the former large 

 quantities of starch and glucose can easily be detected ; the latter 

 are completely wanting in these substances. (The examinations 

 for starch and sugar are to be made when the plants are a few 

 weeks old.) 



These facts, first established by Schimper, find their explanation, 

 as Stahl found, in the fact that the Maize plants which take up 

 Sodium chloride do not assimilate, since their stomata, owing to 

 the influence of the salt, close. The absorption of Carbon dioxide 

 by the plants is then inadequate. By means of the Cobalt test 

 which will be fully discussed in the section on Transpiration, the 

 closure of the stomata can in fact be easily determined. 1 



1 See Stahl, Botan. Zeitung, 1894; Nagamatsz, in Arleiten d. botan. Instituts 

 in Wiir~lurg, Bd. 3 ; Schimper, Sitzunysber. d. Akadem. d. Wiss. zu Berlin, 

 1890, Sitzung V., 31 July. 



