began to develop into a clone of second genera- 

 tion Blastocladiellas , and therein the totipo- 

 tency of this organism began to manifest itself. 

 There appeared, gradually, big cells and small 

 cells, brown, colorless and orange cells, cells 

 with thick walls - some sculptured and some 

 not - and cells with thin ones, dead cells and 

 living cells, cells which discharged yet another 

 generation of spores among which no spores 

 were viable, or in which some or all were 

 viable. This, then, will suffice to show the gen- 

 eral nature of the problem as it appeared at 

 the outset in the late 1940's. 



During the past two decades, some -but by 

 no means all- aspects of this picture have been 

 unscrambled and clarified. Let us deal with 

 one of these, now, by way of Fig. 2. We know 

 that the spore with which B. emersonii begins 

 its life history is motile, uniflagellate, uninucle- 

 ate and organized in a very crafty and intriguing 

 fashion; its internal structure is unique. Indeed, 

 its architecture has a direct and important bear- 

 ing upon the story I will try to develop. However, 

 inasmuch as Dr. Lovett will devote all of his 

 talk to the spore and its activities, I will by-pass 

 it without further comment. In synchronized 

 single generation cultures, some 99% of the 

 germlings of B. emersonii can develop along 

 either of two major morphogenetic pathways. 

 Along both of these paths, the thallus increases 

 at an exponential rate in dry weight, volume and 

 other features. A point is reached, however. 



OC pathway 



motile 

 spore 



r 



I S^o-^Q-^O ^''a°ow"th'' differentiation sporangium 



^ J^ stage "°^' germinationrj^ 



germination /:i^^^. /^ Miv^"^ 



"°^^ I RS pathway 



Fig. 2. 



The two major developmental pathways taken by Blasto- 

 cladiella emersonii. (Fig. 2, Cantino and Lovett, In "Adv. 

 In Morphogenesis," III, 1964; reproduced with permis- 

 sion of Academic Press.) 



when this exponential phase in development 

 ceases; at this point, the plant embarks upon its 

 second developmental stage, i.e., cell differen- 

 tiation. The first visible evidence that this is 

 about to happen is the formation of the septum, 

 which delimits the thallus into a large upper cell 

 and a small lower cell with root-like rhizoidal 

 appendages. At this point, it is important to 

 call attention to the fact that at maturity, the 

 lower cell is (as far as is known) devoid of con- 

 tents. The upper cell possesses all the cyto- 

 plasm and all the nuclei - hundreds to thousands 

 of them, depending upon the environmental con- 

 ditions selected - which are embedded therein. 

 Thus, it should be recognized at the start that 

 we are dealing here with the development of a 

 single cell, for we are starting with a uninu- 

 cleate spore and ending with a multinucleate 

 coenocyte. Finally, however, at the very end of 

 this organism's life history, these many nuclei 

 embedded in a common cytoplasm do become 

 delimited from one another by the formation of 

 cross walls - or, perhaps more appropriately, 

 cross "membranes." In any case, each nucleus 

 functions as a focal point for the formation of a 

 spore. Each nucleus inherits a tail, a mitochond- 

 rion and a number of other organelles, and the 

 lot becomes surrounded by the flexible spore 

 wall. This population of nuclei is then liberated 

 from the parent cell in the form of a population 

 of spores, and we are back where we began. 



Now, let me retrace my steps for just one 

 moment. Long ago, we labeled the uppermost 

 cell in the top pathway (Fig. 2) an ordinary 

 colorless cell, or an "OC" cell for short. The 

 uppermost cell in the lower pathway is a re- 

 sistant sporangial cell, or an "RS" cell for short. 

 These two cell types differ from one another in 

 many ways, some very obvious but others de- 

 ceptively subtle. The more obvious differences 

 include the following: the OC cell wall is 

 chitinous and thin, while the RS cell wall, also 

 chitinous, is much thicker; the OC cell wall is 

 essentially colorless, while the RS cell wall is 

 brown, being impregnated with melanin. The 

 protoplast of the OC cell contains no detectable 

 colored carotenoids, while that of the RS cell 

 contains gamma-carotene. There are many, 

 many other differences, but these are too num- 

 erous to mention now. 



Finally, we also learned that these develop- 

 mental pathways could be controlled at will by 

 the simple expedient of providing the organism 

 with a little bit of baking soda. In the presence 

 of a suitable amount of exogenous bicarbonate, 

 essentially all spores developed along the RS 



150 



