448 VEGETABLE PHYSIOLOGY 



its way into the soil by virtue of its geotropism and aphe- 

 liotropism, aided by the movement 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 in the higher plants 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. 



There is another kind of apogamy known, which is 

 generally 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 normally exhibit the fusion of particular hyphae 

 and the admixture of their contents, or gametes, a variation 

 of the process is observed which comes under this category. 

 Instead of two gametangia meeting, and their contents 

 fusing, to form the zygospore, these organs are de- 

 veloped singly and do not coalesce. In this case the 

 fertile cell, which should be a zygote, is produced parthe- 

 nogenetically in each, and is known as an azygospore. 

 Another variety of parthenogenesis, which resembles the 

 apogamy of the Ferns, occurs in Ccelebogyne, where an 

 embryo is produced in the embryo-sac but without pollina- 

 tion or fertilisation. No sexual cell is produced, but there 

 occurs a vegetative budding of one or more of the cells 

 of the nucellus of the ovule, the buds growing into the 



