BILLINGS: EMBRYO NUTRITION IN LABIAT^E. 79 



without special absorptive structures, may be regarded as a 

 simpler, less differentiated, perhaps more nearly primitive, 

 sort. 



In some instances the innermost cell layer of integument, 

 which borders on the nucellus or else on the embryo sac, is 

 specialized into a digestive layer, whose activity is directed 

 against the remaining integument for the benefit of the em- 

 bryo sac. The name "tapetum" has been frequently applied to 

 this layer, but its use is justified only on account of the nutri- 

 tive function it performs. GoebeP proposes the term "epithe- 

 lium" as a substitute. Three characteristics usually pertain to 

 the epithelium: first, it remains intact for a longer period 

 than the integumental layers adjacent; second, its cells be- 

 come differentiated in form or contents from those of the re- 

 maining integument ; third, it apparently exerts a digestive ac- 

 tion. The digestive action is doubted by some, among whom is 

 Schmid," who ascribes a protective function to it. Since the 

 species of Labiatse herein described do not develop an epithe- 

 lium, it is without the province of this discussion to do more 

 than call attention to the fact that Schmid challenges the re- 

 sults obtained by the "Goebelische Schule," thus showing that 

 the problem is open for further investigation. 



The Labiatse, in common with some other sympetalous fami- 

 lies, depart from the type described for many plants having 

 two integuments by differentiating haustoria. The haustorium 

 implies increased absorptive activity in one portion of the 

 embryo sac, but not necessarily cessation of the function else- 

 where. Among the labiates studied, all (with the single ex- 

 ception of Salvia) have a micropylar haustorium of some sort. 

 In most of the species it is a well-developed structure, exhibit- 

 ing manifest activity as a nutritive organ. But in such species 

 as Scutellaria parvula, Teucnum canadense, and Lycopus ru- 

 bellus, it is of small size, and its usefulness may with safety 

 be said to be either greatly reduced or else obsolete. Whether 

 such a condition really represents actual reduction from a pre- 

 viously more highly developed state, or whether it is primitive 

 in its nature, is a problem yet to be solved. The much elon- 

 gated suspensor is here offered as evidence favoring the former 

 view, since the purpose of elongation is believed to be the re- 

 moval of the embryo from the region of the haustorium. Ex- 

 traordinary length in proportion to the size of the haustorium 



