INTRODUCTION 249 



division walls are no longer strictly transverse but oblique (Fig. 46, /). Finally, 

 especially under high temperatures, cell division ceases entirely, though growth 

 and nuclear division continue and giant cells with many nuclei result (Fig. 46, 

 //, ///). We have now obtained forms which are no longer typical, since they 

 have lost their power of normal development and cannot regain their original 

 shape. Shapes not less remarkable, but still normal, may be obtained by 

 a qualitative alteration of the nutritive solution, viz. by retaining sugar as the 

 source of carbon but'by employing ammonia or some related body (amines) in 

 place of peptone. The cells now become more rounded and divide irregularly 

 in all directions ; further, the walls surrounding the daughter-cells become 

 stratified in a remarkable manner (Fig. 46, IV). Each cell is surrounded not 

 only by its own cell-wall but by that of the mother-cell, and finally by that of 

 the parent cell of all. The membranes then gradually degenerate and the cells 

 become free, separate from each other, and take on a spherical form. This 

 growth form reminds one of certain lower Algae and may, as in that case, be 

 termed the ' palmella ' form. By keeping the nutritive solution constant, 

 Basidiobolus may be made to continue growing in the palmella form for an 

 indefinite length of time. 



Since every cell is physiologically independent, multiplication results from 

 every division ; as is the case with many of the lower organisms, it is impossible 

 to separate vegetation from reproduction. Basidiobolus, however, also forms 

 special reproductive cells, and these have one function only they are not at 

 the same time vegetative. 



Conditions are complicated by the fact that two kinds of reproductive cells 

 are produced ; the one type is the so-called conidium (Fig. 47, //), which arises 

 in the following way. A swelling appears at the end of one of the cells which 

 stand up erect out of the nutritive substratum; into this swelling almost all 

 the protoplasm, together with the nucleus, migrates. The swelling is finally 

 segmented off by a transverse wall and the conidium is complete ; it is after- 

 wards thrown off by a special mechanism. The function of the conidium is not 

 to increase the extent of fungus in the same substratum, but to distribute it to 

 another. The other type of reproductive organ the zygospore arises in quite 

 a different way. Two cells of a filament develop beak-like projections at the 

 limiting wall (Fig. 47, ///), which in that situation become dissolved, so 

 that the protoplasm of one cell, or at least most of it, can wander over into 

 the cavity of the other (IV, V). The protoplasts thus fuse and the product 

 becomes enclosed in a very thick wall, which is always characteristic of this 

 type of spore, and indicates that the cells concerned are able to, or must, pass 

 through a long resting period. The two nuclei of the zygospore fuse later. 

 Zygospore formation is a sexual process of a very simple type, the spore being 

 formed by the fusion of a male (the motile cell) with a female cell. 



Thus Basidiobolus has both sexual and asexual reproductive organs, and 

 these are distinguished not only by their mode of origin but also by the 

 conditions under which they are formed and by their significance in the plant's 

 life-history. It has already been noted that the conidia are formed outside the 

 medium ; the zygospores, on the contrary, are only developed in it. Both struc- 

 tures, however, appear only when the nutritive solution begins to become 

 exhausted. The formation of either type of reproductive organ may be sup- 

 pressed by periodical renewal of the culture fluid. 



Basidiobolus is thus an organism showing very varied shape and modes of 

 development, and the variations observed in it demand our special attention, 

 depending as they do in the most obvious manner on external conditions which 

 are determinable at will. This example teaches us how external factors may 

 influence the shape which the plant assumes. 



It is unnecessary for us to describe all the intermediate stages in plant 

 shapes that exist between a simple fungus and the most highly developed plant, 



