256 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



its cells become filled with aleurone grains, and the walls become 

 slightly thickened. 9 



There is much uncertainty regarding the possible function of this 

 "aleurone layer." According to Haberlandt (1914), its chief func- 

 tion is not that of storage but the secretion of diastase and other 

 enzymes so that the food materials stored in the endosperm may be 

 made available to the embryo in a soluble form. Arber (1934) men- 

 tions experiments in which isolated fragments of the aleurone layer 

 were placed on damp filter paper and then covered with a mixture 

 of starch and water. As a control, a similar mixture of starch and 

 water was laid upon damp filter paper without any aleurone layer. 

 After 24 hours the starch grains of the control experiment were still 

 intact while those kept on the aleurone layer had been corroded and 

 were about to fall into pieces. 



A different kind of differentiation of the outer layers of the endo- 

 sperm is seen in Circaeaster (Junell, 1931). Here the integument 

 is completely used up during the maturation of the seed, and the 

 peripheral cells of the endosperm become suberized to form a pro- 

 tective layer. In Crinum (Tomita, 1931; Merry, 1937), where the 

 ovule is naked (see page 63) and the endosperm ruptures the 

 thin pericarp so as to become completely exposed to the air, the 

 suberization is still more marked, and on being wounded the cells 

 react like a phellogen by undergoing tangential divisions and form- 

 ing additional layers of cells. 



In the Annonaceae, Myristicaceae, and some members of the 

 Palmaceae and Rubiaceae, there is a "ruminate" endosperm. It is 

 said to arise as the result of invaginations of the outer tissues, which 

 penetrate deeper and deeper and eventually appear as dark wavy 

 bands in the mature seeds. 10 In Psychotria, recently studied by 

 Fagerlind (1937), it seems that the rumination is not a passive 

 phenomenon but is due to the activity of the endosperm itself, which 

 grows out and fills the ridges arising in the integument in post- 

 fertilization stages. 



Several workers (see Arber, 1934) have commented on the peculiar 



9 In some plants like Oryza (Juliano and Aldama, 1937) there are two or three 

 layers of aleurone cells. 



10 In most books the invaginations are said to be derived from the perisperm, but 

 this needs to be verified by careful developmental studies. As stated in Chap. 3, 

 the nucellus is often so ephemeral that there is no perisperm in postfertilization 

 stagcfc 



