KEPRODUCTION 459 



the nutritive matter is stored outside the embryo. In 

 both cases the root makes its way into the soil by virtue 

 of its geotropism and apheliotropism, aided by the move- 

 ment of circumnutation, and by the adhesion of the root- 

 hairs to particles of the soil. 



In some Monocotyledons the upper part of the single 

 cotyledon remains in the seed and absorbs the nutriment 

 from the endosperm, while its base elongates and thrusts 

 the young plant downwards. 



Sometimes the usual alternation of sexual and asexual 

 reproduction is interfered with by the substitution of the 

 vegetative method for one of them. In the phenomenon of 

 apospory, noticeable in some Ferns, we have small prothallia 

 developed on the back of the leaves in the place of spores. 

 This is a case of the production of a bud instead of an 

 asexual cell. Apospory is also known to occur among the 

 Mosses. 



In the Ferns, again, the sporophyte sometimes arises 

 as a bud or vegetative outgrowth upon the prothallium, a 

 phenomenon known as apogamy. 



A curious phenomenon is occasionally met with which is 

 termed parthenogenesis. It occurs among the Fungi, where, 

 as in Saprolegnia, oospheres are formed in oogonia, which 

 do not become fertilised, and yet have the power of growing 

 out into new plants. In some species of Mucor which nor- 

 mally exhibit the fusion of particular hyphae and the admix- 

 ture of their contents, or gametes, a variation of the pro- 

 cess is observed which comes under this category. Instead 

 of two gametangia meeting, and their contents fusing, 

 to form the zygospore, these organs are developed singly and 

 do not coalesce. In this case the fertile cell, which should 

 be a zygote, is produced parthenogenetically in each, and is 

 known as an azygospore. Another variety of partheno- 

 genesis, which resembles the apogamy of the Ferns, occurs 

 in Ccelebogyne, where an embryo is produced in the embryo- 

 sac, but without pollination or fertilisation. No sexual cell 

 is produced, but there occurs a vegetative budding of one or 



