REMOVAL OF CARBOHYDRATES FROM LEAVES 293 



suit, and when translocation has been going on for some time in the 

 absence of photosynthesis (in darkened leaves), transitory starch 148a is no 

 longer to be found anywhere, except in the conducting parenchyma. 

 At this stage the iodine test produces a blue-black network upon a 

 yellow ground. Finally, the bundle-sheaths also become emptied ; in 

 their case the process of depletion begins with the smallest bundles 

 and gradually extends to the larger veins, while it simultaneously 

 advances in the leaf as a whole in the basipetal direction. According 

 to Schimper, the course of depletion can be very readily observed in the 

 leaves of Hydrocha/ris Morsus ranae. 



In many plants perhaps in the majority the migrating carbo- 

 hydrate material may be temporarily converted into starch in any cell 

 through which it happens to be passing. This so-called transitory 

 starch is found to some extent even in the internal layers of the 

 palisade-tissue, but is abundant in the spongy parenchyma, and 

 accumulates in even larger quantities in the conducting parenchyma, 

 where it is also most persistent. In certain cases, on the other hand, 

 the migrating carbohydrates always occur in the soluble form as glucose, 

 either from their first formation, that is to say, even in the palisade-cells 

 (spp. of Allium), or at any rate from the bundle-sheaths onwards 

 {Impcdiensjparvijiora according to Schimper). 



The solution of the surplus starch that accumulates in the photo- 

 synthetic system, is brought about, according to the detailed observations 

 of Brown and Morris, by the action of diastase, just as in the case of 

 the starch deposited in storage tissues (cf. Chap. VIII.). 



IV. INFLUENCE OF LIGHT UPON THE DISTRIBUTION 

 AND ORGANISATION OF THE PHOTOS YNTHETIC 



SYSTEM. 



The synthesis of organic substance from carbon-dioxide and water is 

 well known to be dependent upon the access of light. In this respect 

 the requirements of different species vary considerably ; many shade- 

 plants in particular are satisfied with a somewhat scanty supply of light. 

 Nevertheless, every plant endeavours to arrange its photosynthetic 

 tissues in such a manner that they will obtain the most favourable 

 illumination. Hence light-intensity is the factor which primarily deter- 

 mines the location of the photosynthetic system, invariably causing it 

 to take up its position as near the periphery as possible, no matter what 

 special type or modification is in question. This fact is most evident in 

 the case of those cylindrical or prismatic photosynthetic organs which 

 at the same time contain voluminous water-storing tissues, and which 

 are devoid of sharply differentiated "light" and "shade" surfaces. Even 



