1402 A TEXTBOOK OF THEORETICAL BOTANY 



sporial cell. This, in different genera, may produce two megaspores of 

 which the upper one develops; or four megaspores, of which either the 

 top or bottom one may develop; or one megaspore which cuts off a parietal 

 cell before developing into an embryo sac; or four megaspores in the trans- 

 verse plane, of which one develops, cutting off a parietal cell beforehand ; 

 or, finally, no megaspore, the primary archesporial cell forming an embryo 

 sac directly. 



The first division is always transverse to the nucellus, forming a dyad 

 of cells. This may end the process in some plants, two megaspores only 

 being formed {Allium, Commelina), but this is rare. Three is a more common 

 number, due to one of the first two cells failing to divide, although its nucleus 

 may divide {Yucca, Iris, Caltha). All gradations are found between an 

 undividing mother cell {Lilium, Fritillaria, Piper) and the regular group of 

 four megaspores. Even double tetrads with eight spores have been found. 

 There is no discernible relationship to the systematic status of the plant, 

 nor any tendency towards suppression of megaspore development in more 

 advanced families. Indeed the records of numbers found are of only minor 

 interest, since the number and arrangement of the spores may vary in one 

 species, or even, at different times, in one individual. An exception to this 

 instability is provided by the Metachlamydeae in which the full complement 

 of four spores is almost universal. 



While the linear arrangement is most general, either the upper or lower 

 dyad cell may divide longitudinally instead of transversely, yielding either 

 T-shaped or T,-shaped groupings. The truly tetrahedral grouping has 

 seldom been seen. It is reported as characteristic of Fatsia japonica (Aralia- 

 ceae) but in other species it only occurs as an anomaly. 



The general rule is that only one of the megaspores develops to form an 

 embryo sac, the others disintegrating and being more or less completely 

 resorbed. It is usually the chalazal member of the group which develops, 

 whether for nutritional or other reasons is not clear. The exceptions to this 

 are few. The micropylar spore generally develops in members of the Ona- 

 graceae and in some Compositae. In Rosa either the micropylar or the 

 second spore may develop, while one or two genera are reported in which 

 there is no fixed rule {Gloriosa, Poa). Lastly there are some peculiar cases 

 {e.g., Galium, Sedum, Potentilla) in which there is apparently a competition 

 for nutriment between the four spores, accompanied by extensive outgrowths 

 of haustoria, which seems to determine which spore shall form the embryo 

 sac. Anomalous cases in which two or more spores develop towards embryo 

 sacs are not uncommon. They seldom complete their development, as 

 one usually prevails over the others. 



The condition in Casuarina is highly peculiar and the only appearances 

 at all like it are found in Quercus. In the former genus there are several 

 hypodermal archesporial cells, which divide repeatedly to form longitudinal 

 rows of cells, the upper members of which seem to be parietal, while a large 

 number of the lower ones become sporogenous, providing a larger mass of 

 this kind of cell than is to be found in any other plant, Treub estimated 



