348 



PHYSIOLOGY OF NUTRITION. 



rich in starch. These last could not get rid of their carbohydrates 

 in darkness, being, unlike those left on the plant, unconnected 

 with other organs. 



If we apply macroscopic tests to leaves of Impatiens parviflora 

 (this plant often grows wild with us, or it may easily be de- 

 veloped in a somewhat shady place in the garden, from seed), we 

 shall find starch in large quantities in plants which have been 

 exposed to normal conditions of environment. It appears, how- 

 ever (as I have satisfied myself), that the nerves are poor in 

 starch, as compared with the mesophyll, and they consequently 

 assume with Iodine solution a yellow or only faintly bluish colour. 

 If plants of Impatiens are grown in pots, and a few leaves cut 

 from them are placed in darkness, we shall find that the leaves 

 left on the plants, and those removed, are alike devoid of starch 

 after forty-eight or seventy-two hours. In this respect, there- 

 fore, leaves of Impatiens removed from the parent plant, and 

 kept in darkness, behave differently from leaves of Tropasolum 

 similarly treated. The glucose, which is the product of solution 

 of the starch, and which passes away from leaves left on the plant, 

 can be reconverted by the Tropseolum leaves into starch. Leaves 

 of Impatiens are at most to a small extent capable of effecting 

 this reconversion. 



We now prepare a transverse section from the leaf of Impatiens 

 parviflora, and at once see that the mesophyll is differentiated 

 into palisade and spongy parenchyma. The midrib consists, as 

 usually in leaves, of a peripheral layer of elongated cells poor in 

 chlorophyll, and several vascular bundles the bast of which is 

 covered by a starch layer. The layer of elongated cells which 

 encloses the vascular bundles of the thicker and also those of the 

 thinner nerves we may appropriately designate the conducting 

 sheath. 



It has already been mentioned that the nerves, especially the 

 thicker nerves, of the leaves of Impatiens developed under normal 

 conditions of environment are, at all events, poor in starch. If 

 pot plants of Impatiens are placed in the dark for twenty-four 

 houri, and a few leaves are then cut off and tested macroscopically 

 for starch, the scarcity of starch in the nerves is still more clearly 

 brought out. The nerves stand out as a yellow network in the 

 blue-tinted mesophyll, which is still fairly rich in amylum. We 

 leave in darkness for forty-eight hours pot plants of Impatiens, 

 and also leaves cut from these plants and placed in air rich in 



