32 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



occasionally its cells become densely protoplasmic and simulate 

 those of the tapetum. In Lilium there are several middle layers, 

 of which those lying adjacent to the endothecium persist for a long 

 time (Fig. 23), and in Gloriosa (Eunus, 1949) the outermost middle 

 layer develops fibrous thickenings similar to these of the endo- 

 thecium. 



Rarely, a middle layer may be absent as in the anthers of Wolffia 

 (Gupta, 1935) and Vallisneria (Witmer, 1937), but some previous 

 reports of the absence of a middle layer have been shown to be mis- 



A B 



Fig. 26. Anthers, showing microspore mother cells and tapetum. A, Bougain- 

 villea, t.s. portion of anther, showing mitotic divisions in tapetal cells. (After 

 Cooper, 1931.) B, Salvia mellifera, t.s. portion of anther lobe. The tapetal cells 

 lying toward the connective are considerably larger than those on the outer side. 

 (After Carlson and Stuart, 1936.) 



interpretations caused by its ephemeral nature and early disappear- 

 ance. Johri (1934) has demonstrated the presence of a middle layer 

 in Cuscuta where it was formerly reported to be absent (Peters, 

 1908). 



The innermost wall layer or tapetum is of considerable physio- 

 logical significance, for all the food materials entering into the sporog- 

 enous cells must pass through it. 2 Its cells are full of dense cyto- 

 plasm, and at the beginning of meiosis the tapetal nuclei may also 

 undergo some divisions (Fig. 26). 3 Because of these similarities of 



2 Typically the tapetum is a single layer of cells but in Nicolaia and Costus 

 (Boehm, 1931) it is composed of several layers. 



3 Rarely, tapetal nuclei may even pass through a condition resembling 

 the prophase of a meiotic division. Gates and Rees (1921) figure some tapetal 



