EMBRYO SAC (MEGASPORE) 365 



pollination. In most spring-flowering plants the anthers develop 

 as far as the mother-cell stage in the previous autumn, although 

 in the Hazel ripe pollen can be found in the male catkins during 

 midwinter. 



As a general rule the pollen tube traverses the cavity of the 

 ovary, and thus reaches the micropyle of one of the ovules 

 (porogamy, Fig. 213, A). But, in certain trees and shrubs (e.g. 

 Elm, Beech, Hazel), it grows through the placental tissue and 

 enters the ovule near the chalazal end (chalazogamy, Fig. 213, B). 

 The growth of the pollen tube may therefore be compared to that 

 of a fungal hypha, and its power to penetrate tissues, and eventu- 

 ally the megaspore membrane, is doubtless due to an analogous 

 secretion of digestive enzymes. 



The downward growth of the pollen tube, from the stigma 

 into the ovary, is mainly determined by a chemotropic stimulus 

 due to substances contained in the ovules or in the ovary- 

 wall. This fact can be experimentally demonstrated by sowing 

 pollen grains of the Wild Hyacinth (Scilla) in a 5 per cent., 

 or of Echeveria retusa in a 15 per cent., solution of cane-sugar 

 around a fragment of the ovary, taking care to avoid the inclusion 

 of air-bubbles. Observed under the microscope, the pollen tubes 

 tend to grow in the direction of the piece of ovary. 



The first steps towards the development of the megaspore 

 are to be found in very young ovules, usually before the integu- 

 ments have become plainly differentiated (Fig. 207, A). As in 

 the case of the stamens, the archesporium (ar.) arises subepidermally, 

 but here it usually comprises but a single cell situated near the 

 apex of the nucellus. This divides tangentially (Fig. 207, A, ar.), 

 the inner half generally constituting the mother-cell, which as 

 a rule, by two successive divisions, of which the first is the 

 reduction division, gives rise to a row of four spores, forming a 

 tetrad within the nucellus. It is usually the innermost member 

 of the tetrad that becomes the functioning megaspore (generally 

 known as the embryo sac), this cell subsequently increasing 

 greatly in size so as to occupy the greater part of the nucellus 

 (Fig. 206, e.s.). In the Mistletoe-family (Loranthaceae) no proper 

 ovules are differentiated ; a parenchymatous outgrowth, which 

 arises from the base of the ovary and almost fills the latter, 

 develops a number of archesporia, from each of which an embryo 



