Lecture XIX. 155 



six layers of thin-walled cells of irregular shapes. Near the upper 

 epidermis the mesophyll cells fit more closely together, while in 

 the neighbourhood of the lower epidermis they are very loosely put 

 together and leave between them large spaces. There is no sharp 

 division between these two layers in this leaf as there is in many 

 others ; but notwithstanding this, they may be called the palisade 

 and spongy parenchyma respectively. The spaces between the 

 cells of the spongy parenchyma and the smaller ones between those 

 of the palisade layer form a continuous system of irregular air- 

 passages ramifying throughout the mesophyll, and communicating 

 with the outer air through the stomata in the lower epidermis. In 

 this way the greater part of the surface of each mesophyll cell is 

 exposed to the air and is free to interchange gases with it. The 

 thin-walled cells of the mesophyll contain a lining of protoplasm 

 carrying in it very large numbers of chloroplasts. The central 

 space of each cell is occupied by a large vacuole ; and the osmotic 

 pressure of its dissolved substances bearing on the protoplasmic 

 lining distends the cell- walls and keeps the cells stiff or turgid. 



The conducting tracts emerging from the mid-ribs penetrate 

 the mesophyll. They travel in the upper region of the spongy 

 parenchyma about half-way between the upper and lower epidermis. 

 The structure of the tracts is similar to that we have already 

 studied in the stem. The number of tracheids and sieve-tubes is 

 reduced and consequently the tract becomes finer. It is through- 

 out accompanied by and encased in the endodermis and bundle 

 sheath. As it passes out into the mesophyll it is still further re- 

 ducedthe reduction being most marked in the bast on the upper 

 aspect of the tract. Finally the bast on the upper side completely 

 disappears so that near the termination the tract consists of two 

 parallel strands of wood and bast, one above the other, surrounded 

 by the pericycle and endodermis. Near the margin of the pinnules 

 each of these attenuated tracts bifurcates. 



Outside the endodermis the cells of the mesophyll are elongated 

 and accompany the tract as an ill-defined sheath. These cells 

 are poor in chloroplasts and with the conducting tracts consti- 

 tute the veins of the leaf. Fitting closely together they cut off the 

 strand from the intercellular system of the mesophyll. 



Enough of the structure of Aspidium has already been de- 

 scribed to show that this fern is a very complicated and highly 

 developed organism. Its organs are highly specialised to perform 

 various functions and these functions are broadly conditioned by 

 the fact that it is a holophyte which lives upon land. The 

 materials needed for growth are obtained partly by the root-hairs, 



