MORPHOLOGY 93 



Although it is by fertilisation that the further development of the sexual 

 products is made possible, there are exceptional cases in which a gamete or an egg 

 forms an embryo without being fertilised. If development follows in this way from 

 a gamete or egg with the reduced number of chromosomes the phenomenon is 

 called PARTHENOGENESIS. On the other hand the reduction of the number of 

 chromosomes may be suppressed in the development of the egg so that the full 

 number necessary for further development is already present in it. In this case 

 its development is not true parthenogenesis but a vegetative production of an 

 embryo, which has come about through loss of sexual- 

 ity and falls under the conception of APOGAMY ( go ). 

 The cases of production of embryos from unfertilised 

 eggs among the higher plants belong to the latter category : 

 this is known in Antennaria alpina ( 91 ), Alchemilla ( 92 ), 

 Thalictrum purpurascens ( 93 ), Taraxncum ( w ), Hieracia ( 95 ). 

 In lower plants, in some Fungi, e.g. Saprolegnieae, and in 

 one of the Characeae (Chara crinita) ( M ) true partheno- 

 genesis is found. In Marsilia ( 97 ), one of the Hydro- FJG 10 o. Chlorophyll 

 pterideae, the segmentation and further development of grains from the leaf of 

 unfertilised eggs has also been observed. Funaria hygrmnetriva, 



resting, and in process 



Multiplication of the Chromatophores. This of division, smaii m- 

 is accomplished by a direct division, as a result eluded stan* grains are 



r . '. present in the grains. 



of which, by a process of constriction, a chro- ( X 540.) 

 matophore becomes divided into nearly equal 

 halves. The stages of this division may best be observed in the 

 chloroplasts (Fig. 100). 



B. Cell Fusions 



The connection of the living protoplasts with one another is less 

 complete in plants than in the animal body. This is evident from the 

 existence of the cell wall which surrounds the vegetable protoplast. 

 Recent researches have, however, shown ( 9S ) that the protoplasts of 

 the plant are united together by extremely fine cytoplasmic filaments, 

 which proceed from the boundary layer of the cytoplasm. Such 

 filaments are mostly confined to the pit-membrane (Fig. 101), but 

 may also penetrate the whole thickness of the cell wall (Fig. 102). 

 The existence of these connecting filaments of living substance 

 between the protoplasts confers an organic unity on the whole body 

 of the plant. 



The members which make up the sieve-vessels, or, as they are 

 commonly called, the sieve-tubes, are united by thicker strands of 

 cytoplasm, which facilitate the transfer of substances through the 

 tube. The transverse walls traversed by these strands of cytoplasm 

 have been referred to above (p. 67) as sieve-plates. The pores 

 attain their greatest diameter in some Angiosperms (Fig. 103). It 

 is worthy of special note that, despite the fact that the nuclei of 

 the sieve-tube segments disintegrate, the cytoplasm, which lines the 



