INTRODUCTION TO THE PROBLEM 



edges of the leaves in ferns, on peculiar lateral appendages in the Psilotales, on the 

 upper surfaces of leaves in the Lycopods or attached to special members known as 

 ' sporangiophores ' of possibly axial nature and aggregated into cones in the Equisetales; 

 lastly, in the ancient and undoubtedly primitive fossil Psilophytales the sporangia 

 terminate the ordinary, dichotomously branched, vegetative axes. 



'Flowers' are absent, the relative inconspicuousness of the spore-bearing members 

 being associated with a complete absence of anything comparable to insect pollination. 

 Instead, the male sexual cells are themselves endowed with active powers of self- 

 motility, and these swimming spermatozoids (see Frontispiece) are the immediate means 

 by which fertilization is brought about. The dependence on free water which this 

 mechanism entails is thought to be a primitive feature, possibly inherited from an 

 algal ancestor and certainly shared by other lowly land plants such as the mosses and 

 liverworts; in the higher plants it has been lost. 



The two sex organs which respectively either liberate the spermatozoids or retain 

 the egg during and after fertilization, are not borne on the same plant as the sporangia 

 referred to above. The sporangia liberate unicellular spores which germinate without 

 the intervention of any sexual process into little plants known as prothalli which are 

 always both smaller in size and surprisingly different in structure from the individual 

 from which the spores were derived. Unlike the parent plant a prothallus is never 

 differentiated into stem, leaves and roots and is devoid of vascular tissue (except in a 

 few abnormal cases). It is generally attached to the soil by delicate unicellular hairs, 

 the rhizoids, and it may or may not be green. Where the green pigment is present the 

 prothallus supports itself by the same physiological mechanism, involving photo- 

 synthesis, as the parent plant; this is the case in Equisetum, most ferns and some Lycopods. 

 Where chlorophyll is absent the prothallus may either live in symbiotic relation with a 

 fungus, as a saprophyte, a mode of life adopted by the prothalli of the Psilotales, 

 many Lycopods and the Ophioglossaceae among the ferns; or the prothallus may 

 be so short lived that it is able to complete its entire development on the food reserves 

 laid up for it in the original spore, this is the case in all the ' heterosporous ' members 

 of the Pteridophyta, namely, Isoetes, Selaginella and the Hydropterideae among the ferns. 



Sooner or later, a prothallus of any of these varied types becomes sexually mature. 

 Antheridia are then produced, either superficially or sunk in the tissue, which will 

 liberate many swimming male gametes if water is provided at the right time. The 

 female gametes or eggs are large, non-motile cells, borne singly in characteristic female 

 sex organs, the archegonia. The basal part of an archegonium, containing the egg, 

 is sunk in the tissue of the prothallus which thereby adds to its protection, the rest of the 

 organ being in the form of a projecting multicellular neck which is capable of opening 

 when wetted, to give free access from the outside world to the naked surface of the egg 

 cell. When the union of the gametes has been achieved, it is immediately followed 

 by germination of the fertilized egg in situ. At first, it remains enclosed and nourished 

 by the prothallial tissue, but as soon as the multicellular embryo has become large 

 enough for the first organ (stem, leaf or root as the case may be) to assume its proper 

 function, the prothallial tissue is broken through and an independent organism with 

 the morphology of the spore-bearing plant is re-established. 



MFC 



17 



