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A TEXTBOOK OF THEORETICAL BOTANY 



Reference should be made here to the occasional persistence of parts of 

 the nucellus, especially its apical cap, during the ripening of the embryo. 

 These cells may divide to form a tissue, called perisperm, which may be 



found, e.g., in Piper, alongside 

 of the embryo and forming part 

 of the nutritive tissue in the 

 seed. In the seeds of members 

 of the Centrospermae the endo- 

 sperm is reduced to a few layers 

 of cells around the embryo, 

 while the main mass of nutritive 

 tissue is perisperm, formed on 

 the concave side of the curved 

 embryo. 



The cells of the mature en- 

 dosperm are generally thin- 

 walled and richly stored with 

 reserve food materials. Of these, 

 starch is the commonest, but 

 oils, fats, proteins and "hemi- 

 cellulose " are characteristic 

 of certain genera and families. 

 Mixed reserves, e.g., oils and 

 proteins, may occur, but starch 

 and oil are always distinct and 

 serve to distinguish seeds of 

 different physiological types. 

 Where starch and proteins are 

 both stored they are usually in 

 separate parts of the tissue, the 

 protein-containing cells often 

 forming a special " aleurone 

 layer " on the periphery, a 

 layer which may, however, have 

 another importance as a source 

 of hydrolytic enzymes during 

 germination. 



The deposits of hemicellu- 

 loses, or more properly polyur- 

 onides, cause great thickening 

 of the cell walls, as in many 

 Palmaceae, Iridaceae, etc. The walls of these thickened cells are penetrated 

 by tubular pits and are often richly provided with protoplasmic con- 

 nections. 



The quantity of stored material is often so great that the other cellular 

 contents may be disorganized or disappear, leaving nothing but a cellular 



Fig. 1342. — Physostegia rirginiana. Late stage of 

 embryo sac with basal caecum filled with 

 cellular endosperm but with none in the 

 lateral caecum or the micropylar portion of 

 the sac. 



