THE FEMALE GAMETOPHYTE 109 



closelv associated. From the data more or less scattered through 

 the preceding and following pages, the various methods by which 

 nutritive supplies are brought into the sac may be grouped to- 

 gether as follows, although the subject is in no condition as yet 

 for satisfactory organization. 



The digestion and absorption of adjacent tissue by the en- 

 larging sac is the most general method of obtaining nutritive 

 supplies. It always occurs to a certain extent, and often is the 

 onlv observed method. The varying amount of tissue destroyed 

 in this way is a thing of common observation. 



The organization of a definite layer or lavers of cells about 

 the embryo-sac in its later stages, which we have called the 

 " nutritive jacket," has not been reported for the Monocotyle- 

 dons, occurs in comparatively few Archichlamydeae, while it 

 seems to be common among the Sympetalae. For the origin 

 and occurrence of this jacket see page 101. Its appearance and 

 function is that of a tapetum, and there seems to be no good 

 reason why it should not receive the name. 



Tracheid-like cells have been reported in the nucellar tissue 

 of Casuarina, Castanea, and Asclepias, but this meager list will 

 doubtless be much increased. That such cells are connected 

 with a nutritive mechanism seems clear, but their rare and 

 feeble development suggests a relic of an efficient ancestral 

 mechanism. The recent discovery (Oliver 104 ) of a Palaeozoic 

 fern with certain resemblances to the Cvcadofilices, in which 

 tracheids replaced the tapetum in the sporangium, may be an- 

 other indication of the former somewhat extensive use of this 

 special form of mechanism. Thick-walled cells often appear 

 in the chalazal region, especially in connection with the pene- 

 tration of the sac. Some are as hard as tracheids, while in 

 other cases the walls have become mucilaginous and swollen. 

 Similar cells also occur wherever haustoria invade tissue in any 

 other region of the ovule or outside of it. 



The aggressive penetration of the chalazal region by the 

 elongation of the antipodal extremity of the sac is very common. 

 This definite antipodal haustorium seems to be nearly always 

 developed when a more or less prominent mass of chalazal tissue 

 occurs. Among Monocotyledons such haustoria are recorded 

 among the Gramineae, Liliaceae, and Scitamineae; among the 

 Archichlamydeae they are known to occur among the Sauru- 



