THE ANGIOSPERMAE 1409 



known to occur also in a few Angiosperms, e.g., in certain species of Hiera- 

 ciutn {H. flagellare and H. mirantiacum), also in Mains, Crepis, Hypericum, 

 Poa and Ranunculus. In these cases although megaspores are formed, an 

 embrv^o sac is produced from a neighbouring somatic cell of the nucellus, 

 which may develop either in replacement of a normal sac, or side by side 

 with it. The nuclei of these sacs are, of course, diploid. 



There yet remains an ultimate step in reduction that, so far as we know, 

 has not been taken, for it will be seen from the above descriptions that there 

 is no case in which the four megaspore nuclei themselves, without any 

 change or division, become the nuclear apparatus of the mature embryo sac. 

 Perhaps this cannot be, but we do not know why. 



The timing of ovular development is often out of step with that of the 

 microspores, as we have previously mentioned. In some early-flowering 

 species, such as Corylus, Salix, Populus and Ulmus, the ovules pass the 

 winter in the form of nucellar primordia and development of archesporium 

 only begins in spring, in some cases only after pollination. The latter is 

 generally the case also among Orchids. 



When there is any well-marked diflterence in the timing of development, 

 the commoner condition is the ripening of the pollen grains before the 

 ovules, e.g., Trillium, Populus, Tulipa, etc. The opposite condition, that 

 of the ovules developing in advance of the pollen, is rare, but a good example 

 is Empetrum, where uninucleate pollen grains were observed to be formed 

 at the beginning of August but division of the nucleus did not begin until 

 the following spring. On the other hand the eight-nucleate embryo sacs 

 were already complete early in August. 



Structure of the Normal Embryo Sac 



The embryo sac is a relatively large structure, limited by a thin cellulosic 

 wall and containing at first eight nuclei of similar size and appearance as well 

 as cytoplasm. The latter is mostly aggregated at the two poles of the sac, 

 surrounding the two groups of four nuclei which are located there, while the 

 central part of the sac contains a large vacuole containing sap with a low 

 osmotic potential and with only a thin, peripheral layer of cytoplasm. When 

 the fertilization stage is approached, this vacuole may become separated 

 into several, while a secondary aggregation of cytoplasm forms around the 

 two polar nuclei, which have then paired at, or near, the centre of the sac. 

 The cytoplasm mav also enclcse a considerable number of starch grains, 

 which disappear after fertilization. They are not included in the endo- 

 sperm, even when this contains starch (Fig. 1301). 



In crassinucellate ovules, the expansion of the embryo sac may be 

 accompanied by the destruction and absorption of some of the inner layers 

 of nucellar tissue, but its expansion is not otherwise restricted. In tenuinu- 

 cellate sacs the entire nucellus may disappear, so that the wall ot the sac 

 becomes contiguous with the inner surface of the inner integument. This 

 is generally cuticularized and may also be formed of a closely packed layer 



