THE ANGIOSPERMAE 



1393 



and resemble glandular cells; occasionally there are air-spaces among them. 



Van Tieghem believed that the hypostase limited the chalazal expansion of 



the embryo sac. Another suggestion is that the 



hvpostase may produce a hormone required by the 



embryo sac. If this were so it is difficult to under- 

 stand why it should not be universal. 



A few plants belonging to widely separated 



families show a somewhat similar modification of 



the apical cells of the nucellus. Van Tieghem called 



this the epistase, but its nature also is unexplained. 



Both structures are well developed in the ovules of 



the Lentibulariaceae. There can be little doubt of 



their nutritive function in this family, as the cells are 



rich in protoplasm, and in some species they grow 



out into elongated haustoria which penetrate the 



nucellus around the fertilized embryo sac. 



A singular phenom- 

 enon is seen in the ovules 

 of many Podostemaceae. 

 The outer integument de- Fig 

 velops first and envelops 

 the nucellus, forming the 

 micropyle. The inner in- 

 tegument develops later 

 and only covers the lower 

 portion of the nucellus. 

 This lower portion breaks 



down into a space containing free nuclei and 

 cytoplasm, called the pseudo-embryo sac, while 

 the upper part of the nucellus, containing the 

 true embryo sac, remains unaffected (Fig. 1294). 

 This hollow suggests an analogy with the chalazal 

 haustoria of many plants and is probably nutritive 

 in function. 



The two genera Trochodendron and Cercidi- 

 phyllum have ovules of a peculiar pattern. 

 According to Van Tieghem they develop laterally 

 instead of terminally on the funicle and they 

 have a sub-chalazal extension, which appears to 

 be the funicle apex and into which the funicular 

 bundle makes a hairpin bend, before ending 

 below the nucellus (Fig. 1295). 



Several other cases of remarkable chalazal 

 developments are known. In Bilbergia (Brom- 



eliaceae) there is a prolongation similar to the above but non-vascular, 



while in Aechmea, a member of the same family, the chalaza of the 



1294. — Mourera 

 fliiviatilis (Podo- 

 stemaceae). Young 

 ovule in longi- 

 tudinal section. 

 Pseudo - embryo 

 sac formed by the 

 breakdown of the 

 lower part of the 

 nucellus. {After 

 Went.) 



Fic;. 1295. — Cerddiphylliiu} 

 jdponiciDii. 0\-ule in longi- 

 tudinal section showing 

 the large chalazal exten- 

 sion. {After Sii'amv and 

 Bailey.) 



