292 PHOTOSYNTHETTO SYSTEM 



the actual efferent tissue of the leaf. The evidence of comparative 

 anatomy suffices for the identification of the paths of translocation in 

 photosynthetic organs, since it proves that the migration of synthetic pro- 

 ducts is necessarily confined to certain definite tracts, on account of the 

 general anatomical construction of these organs ; nevertheless the fact 

 that other investigators have arrived at similar conclusions concerning 

 the conduction of carbohydrates in foliage leaves, on experimental 

 grounds, provides a highly satisfactory confirmation of the author's 

 views. 



With regard, first of all, to the assumption that synthetic material 

 is removed as quickly as possible from the palisade-tissue, we may 

 quote an observation, made by Sachs, upon leaves which he tested with a 

 solution of iodine, after they had been killed and decolourised by boiling 

 in alcohol. Sachs states, that if the leaves, before being tested, " had 

 not yet accumulated the full amount of starch, or had, on the other 

 hand, already lost a portion thereof, they appeared brown, or at most 

 brownish black, on the upper side, in cases where the lower half of the 

 mesophyll was coal-black, or even metallic black in colour." The con- 

 clusion to be drawn is, of course, not that the dorsal palisade-tissue has 

 a smaller photosynthetic capacity than the ventral spongy parenchyma, 

 but rather that the synthetic products are removed more rapidly from 

 the former tissue. In other words, the included starch-grains of 

 chloroplasts merely represent the temporary surplus which results, 

 when photosynthesis produces more carbohydrate material than can 

 be removed by translocation in a given time. The author himself has 

 observed an extraordinarily rapid depletion of photosynthetic cells in 

 a representative of the girdle-type of leaf-structure. A leaf of Sac- 

 charum ojflcinartcm picked just after sun-rise, at Buitenzorg, contained 

 no starch, either in the radially elongated photosynthetic girdle-cells, 

 or in the elements of the parenchymatous bundle-sheaths which are 

 likewise well provided with chloroplasts ; in other words, the entire 

 synthetic products [of the previous day] had been removed during the 

 night. A leaf of the same plant picked shortly before the usual daily 

 thunderstorm about 3 p.m., after a sunny morning, also contained 

 no starch in the specialised photosynthetic cells ; in this case, however, 

 the chloroplasts of the bundle-sheaths enclosed large numbers of minute 

 starch-grains. Evidently the migration of carbohydrates from the 

 photosynthetic cells had been so rapid, that no surplus remained in 

 the form of starch, although photosynthesis must have been proceeding 

 briskly all the time. 



When, therefore, a leaf has become heavily charged with starch after 

 energetic photosynthesis, it is clearly the specialised photosynthetic 

 tissue that is relieved first of all ; the spongy parenchyma next follows 



