THE ANGIOSPERMAE 1381 



Whether the single archesporial cell is ever formed at a deeper level in 

 the nucellus is not clear, but many cases are known in which there appears 

 to be an archesporium of several or of many cells, among which some may 

 have come from layers below the hypodermal layer. Pluricellular arche- 

 sporia occur in many families of Dicotyledons, but only rare and occasional 

 cases have been found in Monocotyledons (Liliaceae). In the former group 

 the condition is most widespread in the Fagaceae, Salicaceae, Betulaceae, 

 Corylaceae, Ranunculaceae and Rosaceae. As these families are all low in 

 the evolutionary scale, it might appear that a pluricellular archesporium was 

 a primitive feature among Angiosperms. Comparison with the micro- 

 sporangium also suggests that this is so. Pluricellular archesporia are also 

 found, however, in certain genera of advanced families such as Asclepiad- 

 aceae, Loranthaceae, Rubiaceae and Compositae, though in none of these 

 sufficiently widespread to be reckoned a family characteristic, as in the 

 first-mentioned families. It is not, in fact, certain in all these cases that 

 the tissue concerned is the archesporium itself and not a mass of primary 

 sporogenous cells derived from an archesporium. In Casuarina, for example, 

 the number of archesporial cells is small but they give rise to a large group 

 of sporogenous cells which occupies all the centre of the nucellus. These 

 in their turn develop into very numerous potential megaspores, almost all 

 of which are sterile. 



To return to the development of the ovule, we see that no sooner has the 

 archesporium appeared than the first or inner integument also appears, 

 beginning in the form of a ring-like swelling around the nucellus. When the 

 latter is large the integument arises at or near its base, but in some ovules 

 with a small nucellus it may arise near the apex, or at least it makes its 

 appearance there, as a distinct structure. The outer integument usually 

 appears later and lower down than the inner, and generally develops more 

 slowly (Fig. 1284). 



There is much variation between families with regard to the presence of 

 two integuments. The number is generally constant throughout a family, 

 with few exceptions. Two integuments occur in most families of the Archi- 

 chlamydeae and of the Monocotyledons. Exceptional families which are uni- 

 tegminate are several of those grouped as " Amentiferae ", i.e., Betulaceae, 

 Salicaceae, Myricaceae and Juglandaceae, also the Lauraceae, Cornaceae, 

 Umbelliferae and some of the Ranunculaceae {e.g., Ranunculus and 

 Anemone) and Rosaceae. Exceptional unitegminate genera are also Escal- 

 lonia and Hippuris. Among Monocotyledons, Crinum is reported to have 

 no integuments at all. 



One integument is characteristic of the Metachlamydeae, where it is 

 usually massive. Exceptions which are bitegminate are the orders Primu- 

 lales, Plumbaginales and Ebenales, all relatively primitive members of the 

 Metachlamydeae. 



It is the general opinion that the unitegminous condition has arisen from 

 the bitegminous. The change may have happened in several evolutionary 

 lines. It may have been brought about in two ways: either by coalescence 



