THE MEGASPORANGIUM 69 



Among recent records, in Agave (Grove, 1941) and Strombosia 

 (Fagerlind, 1947), the vascular strand of the ovule is said to pene- 

 trate into the nucellus up to the base of the embryo sac, and in 

 Magnolia (Earle, 1938) it gives out short branches in the chalaza, 

 one of which is directed towards the embryo sac. In Acalypha 

 (Landes, 1946) the main bundle of the ovule proceeds up to the hy- 

 postase and forms a number of short branches whose ultimate rami- 

 fications extend into the nucellus up to a distance about one-fifth 

 of the length of the ovule. More striking still is the condition 

 recently reported in Casuarina (Swamy, 1948), where the funicular 

 strand extends up to the base of the sporogenous tissue, some of 

 whose cells elongate and themselves assume a conducting function 

 instead of giving rise to embryo sacs (Fig. 51). 



The occurrence of vascular elements in the nucellus is of con- 

 siderable theoretical importance, as such a condition has been consid- 

 ered by some authors to be a relic of the highly developed "trachei- 

 dal envelope" found in some fossil gymnosperms. A few years ago 

 integumentary vascular bundles were considered to be very un- 

 common in angiosperms, but now they are known to occur in several 

 families. Possibly the occurrence of xylem elements in the nucellus 

 may also be found to be more frequent than the few reports just 

 mentioned may seem to indicate. 



Archesporium. The archesporial tissue is of hypodermal origin. 

 In general, one cell of the nucellus, situated directly below the epi- 

 dermis, becomes more conspicuous than the others owing to its larger 

 size, denser cytoplasm, and more prominent nucleus. This is the 

 primary archesporial cell. Frequently the cells situated below it 

 lie in a row so that the archesporial cell appears as the terminal 

 member of a series of nucellar cells (Fig. 43 A). 



The archesporial cell may divide to form a primary parietal 

 cell and a primary sporogenous cell (Fig. 49A-B), or it may func- 

 tion directly as the megaspore mother cell (Fig. 50H). The primary 

 parietal cell may remain undivided or it may undergo periclinal and 

 anticlinal divisions to form a variable number of wall layers. The 



ments in the nucellus or inner integument of the Thymelaeaceae are due to mis- 

 interpretations. In his opinion these tracheids really belong to the chalazal tissue 

 which, by "vigorous growth," extends around the endosperm and thus forms a 

 part of the seed coat. Fuchs (1938) and Kausik (1940) also failed to observe any 

 nucellar tracheids in the species studied by them. 



