272 SCIENCE PROGRESS 



for the guard cells of the stomata the epidermis is usually with- 

 out chlorophyll. 



Beneath the upper epidermis are the " palisade cells," elon- 

 gated at right angles to the surface of the leaf, fairly closely 

 packed and in one or more layers. They contain numerous 

 chlorophyll granules and are the cells in which the formation of 

 carbohydrates proceeds most actively. Frequently they are 

 arranged in groups with their lower ends connected to a common 

 cell termed by Haberlandt a " collecting cell," for the reason 

 that such a cell must receive whatever products pass out of the 

 group of palisade cells above it. (Figs, i, 15, 16.) 



Beneath this somewhat regular palisade are frequently to be 

 found very loosely packed, irregular cells, which anastomose in 

 all directions and form the ** spongy mesophyll," a tissue full 

 of large intercellular spaces. (Figs, i, 16.) The collecting cells, 

 when present, are in connection with this spongy mesophyll, 

 which thus forms the next link in the chain of elements between 

 the places of formation and of utilisation of the assimilates. 

 When occurring in large amount the spongy mesophyll presents 

 a maze of paths leading in all directions. 



The ParcHchymatoits Bundle Sheath. — Many of the cells of the 

 spongy mesophyll are united to the sheaths of thin-walled and 

 often elongated cells enclosing the vascular bundles of the 

 innumerable fine veins which ramify throughout the lamina. 

 The cells of the sheath form a distinctly characterised tissue 

 designated by Schimper the " Leitscheide" or conducting sheath. 

 (Figs. I, 9-13.) 



The structure of such parenchymatous bundle sheaths of 

 the veins of dicotyledonous leaves has been investigated by 

 Schubert, whose results will be considered presently. 



It is quite obvious that this sheath of the finer veins must 

 receive the assimilates from those cells of the spongy mesophjdl 

 with which it is connected ; but it was thought by Haberlandt 

 and Schimper that the sugars and other easily diffusible 

 substances were conducted from the leaf in the sheath and the 

 parenchymatous tissues continuous with it. 



This belief was based upon a number of anatomical observa- 

 tions and also upon the results of certain microchemical 

 investigations made by Schimper. The latter, however, made 

 one serious omission in that, save for an uncritical experiment 

 upon Plantago, he apparently failed to take into account the 



