THE ENDOSPERM 255 



embryo take place rapidly have a free nuclear endosperm. Others, 

 in which the growth is slow or the mature seed contains only an 

 undifferentiated embryo, have a Cellular endosperm or show cell 

 formation at a very early stage. Numerous examples can, however, 

 be cited in which there is no such correlation. In Impatiens, cited 

 by Rao in support of this theory, the endosperm is not Nuclear as 

 stated by him but Cellular (Dahlgren, 19346). 



Finally the question arises as to which of the two types, the Nu- 

 clear or the Cellular, is the more primitive and which is the more 

 advanced, but to this we have no definite answer. Coulter and 

 Chamberlain (1903) suggest that the Cellular type is the more 

 primitive, since "even when the endosperm begins with free nuclear 

 division, a rudimentary cell-plate often appears, suggesting deriva- 

 tion from an endosperm in which nuclear division was followed by 

 cell-formation." Schiirhoff (1926), Ono (1928), and Glisic (1928) 

 have supported this view. Most other authors, however, consider 

 the reverse derivation, i.e., from the Nuclear to the Cellular type, 

 to be the more plausible (Schnarf, 1929). However, this is still a 

 debatable question, since both types occur side by side in the most 

 primitive orders (e.g., Ranales) as well as the most advanced (e.g., 

 Campanulales). 86 Further, in the Rubiaceae and Orchidaceae also, 

 which are generally admitted to be among the most highly evolved 

 families, we have a free nuclear endosperm. 



Histology of the Endosperm. The cells of the endosperm are 

 usually isodiametric and store large quantities of food materials 

 whose exact nature and proportions vary from one plant to another. 

 As a rule the walls are thin and devoid of pits, but when hemicellu- 

 lose is the chief food reserve they are greatly thickened and pitted. 

 In such cases they may be more or less homogeneous as in Phytele- 

 phas, or show a distinct stratification as in Fritillaria. The pit 

 canals are sometimes very long and their mouths dilated in a trum- 

 pet-shaped manner. Worthy of note also are the plasmodesma 

 strands so clearly seen in the endosperm cells of several plants 

 (Jungers, 1930). 



In the grasses and some other plants, the peripheral layer of the 

 endosperm functions like a cambium and produces on its inside a 

 series of thin-walled cells which become packed with starch. As 

 the seed approaches maturity, the outermost layer ceases to divide, 



86 See in this connection De Vos (1948, 1949) and Stenar (1950). 



