LECTURE XVI 



FERTILIZATION IN PLANTS AND UNICELLULAR 



ORGANISMS, AND ITS IMMEDIATE 



SIGNIFICANCE 



Fertilization in a lichen, Basidiobolus — In Phanerogams — Here too there is reduction 

 of the number of chromosomes by a half — ' Polar cells' in lower and higher plants — 

 Conjugation among unicellular organisms — Noctiluca — The maternal and paternal 

 chromosomes remain apart — Actinophrys — Infusoria — Sexual differentiation of the 

 two conjugates in Vorticella — Importance of the process of Amphimixis — Not a ' life- 

 awakening' process — May occur independently of multiplication — The Rejuvenescence 

 hypothesis — Pure parthenogenesis — The cycle idea — Does Amphimixis prevent natural 

 death ? — Maupas' experiments with Infusorians — Butschli's view — Potential immor- 

 tality of unicellular organisms — The immortality of unicellular organisms and of the 

 germ-cells depends on the fact that there is no time-limit to the multiplication of the 

 smallest living particles — Parthenogenesis is not self-fertilization — Petrunkewitsch's 

 observations on the ova of bees — Is the chromatin really the ' hereditary substance ' '? 

 — Nageli's conclusion from the difference in size between ovum and spermatozoon — 

 Artificial division of Infusorians — Boveri's experiments with the fertilization of pieces 

 of ova not containing a nucleus — Fertilization gives an impulse to development even 

 to non-nucleated pieces of ova — Merogony — The female and male nuclear substances 

 are essentially alike — Summary. 



I now turn to the consideration of the process of fertilization in 

 plants and unicellular organisms. 



With regard to plants, it can now be definitely asserted that in 

 them, too, fertilization is essentially a conjugation of nuclei ; it 

 depends on the union of the nuclei of the two ' sex-cells.' These 

 sex-cells are usually very small among lower plants, indeed up to the 

 phanerogams ; this is especially true of the zoosperm-like male germ- 

 cells, but it usually holds also true of the ovum, which is but seldom 

 burdened with an abundant supply of yolk. In spite of the many 

 difficulties which this smallness of size puts in the way of observation, 

 the untiring exertions of a host of excellent investigators have 

 succeeded in following the process of fertilization in all the larger 

 groups of plants — in algye, fungi, mosses, ferns, and horse-tails 

 among cryptogams, and in phanerogams. 



I shall first give an example from among the lower plants 

 (Fig. 81). In one of the lichens, Basidiobolus ranarum, each of two 

 adjacent cells in the fungus-thread gives off a bill-like process, and the 



