62 BOTANY. [chap. ii. 



most numerous on the under surface — the surface point- 

 ing to the earth. A single row of cells lying just below 

 the epidermis of the upper surface of the leaf — the surface 

 pointing to the sky — are closely packed together side 

 by side, and arranged like a palisade with their ends 

 pointing to the epidermis. This layer constitutes the 

 palisade tissue of the leaf, so named on account of the 

 arrangement of the cells already mentioned. The cells 

 of this tissue are richly supplied with chlorophyll and 

 give the deep green colour to the upper surface of the 

 leaf, and their most important function is that of assimi- 

 lation. The cells of the lower half of the thickness of 

 the leaf form a loose spongy tissue with numerous large 

 intercellular spaces, and contain less chlorophyll than the 

 palisade cells, hence the paler colour of the under surface 

 of the leaf. The intercellular spaces contain gases taken 

 in for assimilation by the upper surface of the leaf, and 

 also water vapour that escapes by transpiration from the 

 leaf into the air, hence we observe that there is a division 

 of labour in the work done by a leaf, the upper surface 

 performing chemical work — assimilation — the under 

 surface physical. The fibro-vascular bundles or ''veins" 

 of the leaf are in continuity with those of the branch 

 from which the leaf originates, and form a network in 

 the spongy, lower part of the leaf, and in many plants 

 project from the under surface. The veins consist of 

 phloem and xylem, the latter lies nearest the upper 

 surface of the leaf which it supplies with water that has 

 passed up from the root. The phloem forms the under 

 side of the veins, and conducts from the leaf into the 

 plant during darkness the assimilated material made by 

 the leaf when exposed to light. 



