SECT. II PHYSIOLOGY 283 



But even in the Pteridophyta this relation is inverted, for the fertilised 

 egg-cell requires to develop forthwith, or else it perishes, while the 

 asexual spores can endure a long resting period. 



It is the rule in digenic reproduction that the sexual cells are 

 individually incapable of development; this takes place only after the 

 sexual cells have united. Thus one use of fertilisation lies in the 

 removal of an arrest of growth, though it cannot be said that this was 

 its original and essential significance. It is much more probable that the 

 sexual cells have gradually lost the capacity of independent development 

 since in this way the possibility of fusion was increased. If every 

 sexual cell commenced to grow at once, this would in most cases take 

 place before fusion with another sexual cell could be effected. 



This assumption is supported by the behaviour of some Algae, in which 

 the sexual cells can often germinate independently ; the egg-cells especially may 

 develop without fertilisation. From the analogy with similar cases in the animal 

 kingdom this phenomenon has been termed parthenogenesis. In the primitive Algae 

 parthenogenesis is possible, because in them the incapacity of development of 

 the egg-cell has either not been acquired or is easily removed under special con- 

 ditions. Thus for example in the Alga Protosiphon parthenogenetic development 

 is induced by a high temperature, and the same happens in the case of the ova of 

 some lower animals (Echinoderms) on treatment with certain chemical substances. 

 It may perhajis be assumed that in the eases in which development only takes place 

 after fertilisation the stimulus to development is given by some substance contained 

 in the sperm-cell. 



Among the higher plants also phenomena to which the name parthenogenesis (*-) 

 has been applied occur. Thus the egg-cells of some Compositae, and also of 

 Alehemilla, Thalictrumpurpurascens, Wickstroemia mdica, Ficus Mrta, and Maisilea 

 Drummondii develoji without previous fertilisation. These cases are distinguished 

 from those just described by the egg-cells in question having retained the number 

 of chromosomes characteristic of vegetative cells. They are diploid cells (p. 94) 

 and not fitted for fertilisation. Stkasburgeii terms these cases apogamy, and 

 distinguishes them from parthenogenesis. Winkler speaks of somatic partheno- 

 genesis in contrast to the true or generative parthenogenesis of haploid eggs 

 (cf. p. 96). 



A number of cases of apogamy (loss of sexuality) are naturally grouped here. 

 In the first instance there are those in which an embryo is develojied from a cell of 

 the embryo-sac other than the egg-cell, for instance from synergidae, antipodal cells, 

 or from other nuclei of the embryo-sac. Further there are cases in which the 

 embryo arises from the nucellus or from the inner integument, projects into the 

 embryo-sac, and assumes the form of an embryo. Here also we are dealing with a 

 purely vegetative reproduction which only resembles the sexual reproduction in 

 the place where it occurs. This development of "adventitious embryos" is as a 

 rule associated with polyembryony, i.e. the development of numerous embryos in 

 seed {Funkia ovata [Fig. 219], Euonymus lal-ifolia, Citrus aurantium, Nothoscordon 

 fraitrans, Caelehogyne ilicifolia, etc). 



The egg-cell previously existing in the embryo-sac is able to continue its develop- 

 ment after fertilisation, but is usually prevented from so doing by the adventitious 

 embryos. The seeds in such rases no longer contain the products of sexual 

 reproduction, but have become organs of vegetative multiplication. The adveu- 



