ii4 THE WHEAT PLANT 



than the inner one and does not reach the micropylar opening. Each of 

 the ovular coats consists of a double layer of cells 14-16 p thick. 



The nucellus has a broad chalaza, and is composed of parenchyma 

 surrounded by a well-defined epidermis of regular closely-fitting cells, 



which are almost square in transverse 

 section and uniformly about 8 p, in 

 diameter except where they abut on the 

 micropyle, at which point the layer is 

 thinner. 



Imbedded in it is the embryo-sac, 

 27 mm. long by -16 mm. broad, a 

 large oval or pear-shaped cell whose 

 narrow end is in immediate contact with 

 the thin epidermis abutting on the 

 micropyle ; at other points it is sur- 

 rounded by nucellar tissue usually 6 to 

 8 cells deep. 



In the early stages of the develop- 

 ment of a spikelet of T. vulgare eight 

 or nine flowers can be recognised, but 

 under the ordinary conditions of plant 

 growth in the field none of the spikelets 

 yield this number of ripe grains. So 

 soon as the three or four lowest flowers 

 FIG. 91. Longitudinal section of the have reached a certain stage of develop- 

 inttguments ment, the upper four or five cease to 

 grow and their tissues become depleted 

 of their cell - contents, the material probably going to assist in the 

 nutrition of the rapidly growing flowers below. 



In a very young spikelet all the cells of the tissues up to the apex 

 contain normal cytoplasm and well-defined nuclei ; but in the early part 

 of June, when growth and differentiation of the parts of the flowers are 

 proceeding rapidly, the cytoplasm and nuclei of the cells of the upper 

 parts of the spikelet show signs of change. The cytoplasm loses its 

 staining power and the nuclei begin to degenerate : later, the cytoplasm 

 disappears, the nuclei remaining for a time, after which they also vanish, 

 leaving only the cell walls. 



Depletion of the cells begins first at the undifferentiated tip of the 

 axis of the spikelet and gradually proceeds downwards to a point where 

 the three or four basal flowers are growing normally. 



