284 



BOTANY 



I'ART I 



titious germs in the polyembryonic seed are, liowever, so far dependent upon 

 sexual reproduction, that for the most part they only attain their development in 

 cases where pollination has previously taken place ; but in Caelebogyne, one of the 

 Australian Euiihorhiaceae, of which usually only female specimens are found in 

 cultivation, and in Balanophora clongata and Elatostcma acuniinahnn according to 

 Tkeub, and Bal. globosa according to Lotsy, the adventitious germs develop with- 

 out the stimulus of fertilisation. These plants accordingly afford examples of a 

 well-marked loss of sexuality. 



This is the place also to mention the Ferns (^■^) in which the development of the 

 embryo frequently does not proceed from the fertilised egg-cell. In Lastraea 



2)seudo-mas polydactyla two ordinary 



Fig. 219. — Vegetati\e formation of embryos in 

 Funkiaovata^Hostacoerulea); n, nucellus with 

 cells in process of forming the rudiments (of) 

 of the adventitious embryos ; S, synergidae ; 

 E, egg-cells, in the lower figure developing 

 into au embiyo ; il, inner integument. 



reproduction cannot consist in 



prothallial cells fuse to give rise to tlie 

 sporophyte. In Athyrium filix foemina 

 clarissima, Jones, a prothallial cell with 

 a diploid nucleus develops directly into 

 the embryo without any process effusion ; 

 in N'ephrodium moUe the same takes 

 ])lace with a haploid cell. In Athyrium 

 filix fcmina clarissima, Bolton, and in 

 Marslica Drummondii the sporophytes 

 develop without fertilisation from diploid 

 egg-cells. In many other cases, which 

 are not confined to cultivated forms, the 

 process of apogamous development has 

 still to be studied in detail. 



In Ferns, and occasionally also in 

 Phanerogams, apogamy is found associ- 

 ated with apospory, i.e. the absence of 

 spores ; the protliallus or the embryosac 

 in these cases arise from ordinary vegeta- 

 tive cells of the sporophyte. 



We thus arrive at the con- 

 ckision that the essential of sexual 

 the removal of the arrest to 



development of the sexual cells. This leads us to consider the fusion 



OF THE SUBSTANCE OF THE TWO CELLS AND THE MINGLING OF PATERNAL 

 AND MATERNAL CHARACTERS WHICH FOLLOWS FROM THIS. This brings 



out the chief distinction between the two modes of reproduction ; the 

 vegetatively produced progeny are due to no such mingling of 

 characters. The complex of characters in vegetative multiplication 

 does not differ as a rule from that in the parent form. As a matter 

 of fact, we obtain by vegetative multiplication all the varieties and 

 races of our cultivated plants, even when these do not come true from 

 sexually produced seed. 



It is of course not excluded tliat tlie vegetative descendants like the parent plant 

 or its parts may undergo variation. From a bud of a tree a branch with new 

 characters (differing in form or colour) may arise by what is known as bud-varia- 

 tion ; on such an altered branch years after, a complete or partial reversion to the 

 ancestral cliaracters may take place (atavism). In the same way variations may 



