THE MONOCOTYLEDONOUS PLANT 77 



whole length, intercellular spaces being small and not 

 numerous. There may be only one layer or several 

 layers of these cells, which constitute what is known as 

 the palissade tissue. The cells contain numerous chloro- 

 plasts, which are embedded in their protoplasm. Each 

 has as usual a central vacuole filled with water. These 

 chloroplasts are capable of a little movement in the celL 

 It is in this layer, exposed to the light most freely, that 

 the sugar is constructed. (See Fig. 20, p. 52.) 



The lower half of the leaf is made of cells which are 

 spherical, cubical, or oblong, and are arranged so as to- 

 touch each other only at few points ; consequently the 

 intercellular passages between them are very large, 

 taking up sometimes more space than the actual cells. 

 This is often called the spongy tissue of the leaf. The 

 cells contain some chloroplasts, but not nearly so many 

 as the palissade cells. This layer is the layer in which 

 evaporation occurs. The veins generally run in the 

 centre of the blade, between these two layers of cells. 



All the structures of both petiole and blade thus show- 

 exact adaptation to the two main duties of the leaf. 



CHAPTER VIII 



THE MONOCOTYLEDONOUS PLANT 



OUR attention has been mainly directed so far to the 

 peculiarities of the dicotyledonous plant. We must 

 now turn for a little while to study another form, in 

 which the embryo has only one cotyledon. The plants 

 of this type are not so numerous as the former class, but 

 they are still very widespread. The most easily acces- 

 sible of them in this country are the grasses and the 

 group which is represented by the common white lily. 



If we take a grain of wheat we have what is very 

 generally spoken of as the seed of the plant. This is 



