26 



GENERAL DIFFERENTIATION OF THE PLANT-BODY 



rr 



the stage of formation of the colonies and thai of the formation of spores a vege- 

 tative stage is intercalated ; and this enables the number of spores formed to 

 be greatly in excess of the energids which unite to form the colony, whereas 

 originally, as in Guttulina, the number of spores and of energids is equal. In the 

 most highly developed cases, even in the Acrasieae, a division of labour is found 

 between the energids; a fructification is formed in which all the energids do not 

 become spores, but only a portion of them, usually the larger, is devoted to this 

 purpose, whilst the others are applied to the formation of a stalk. 



Polysphondylium violaceum and Dictyostelium mucoroides, which have been 

 minutely investigated by Brefeld 1 , are very instructive forms. In these species 

 naked amoeboid energids issue from the germinating spore and multiply by 

 bipartition, but do not unite to form a plasmodium as in other Myxomycetes. 

 It is only at the time of formation of the fructification that they aggregate together 



in uncommonly large numbers to form a dense 

 mass, influenced no doubt by chemotaxy. A 

 ' division of labour ' now ensues. All do not 

 become spores. The central portion of the mass 

 of amoebae is devoted to the construction of 

 a cellular stalk ; the amoebae concerned in this 

 absorb water from their neighbours, surround 

 themselves with a membrane and become poly- 

 hedric cells. The stalk grows at its apex by 

 the amoebae at this position becoming trans- 

 formed into stalk-cells. The amoeboid mass 

 creeps up the stalk to its apex, and when the 

 formation of the stalk is completed all the 

 FIG. 3. Pediastrum granuiatum. A colony amoebae which have not been employed in 



all the cells of which are empty excepting four 1-1 11 i TT 



whose contents are forming daughter-colonies. making the Stalk become Spores. Here all 



B young colony with cells sttll irregularly ,. , .... ,., , , . 



placed, c somewhat older colony, tife ceils the amoebae are originally alike, and their 



now arranged in one plane, the outer ones .1 i_ j i j ^L i 



are evidently two-homeS, the inner ones show position in the amoeboid heap and their reciprocal 





influence are the factors determining 



which 



of them shall become stalk-cells and which 



spores. The formation of the stalk is clearly of advantage for the distribution 

 of the spores and it has the same function as in the sporogonium of a moss. 

 One of these fructifications of Polysphondylium, growing at its apex by means 

 of the new energids which creep up to it, furnishes a remarkable counterpart 

 of a shoot of one of the higher plants which also possesses at its apex a vegetative 

 point of embryonal tissue but below is composed of mature energids. 



2. COLONIES OF ENERGIDS INVESTED BY A MEMBRANE. 



A few examples only which are instructive in connexion with general questions 

 of organography will be cited here. 



a, Protococcaceae. Figure 3 is a representation of a colony of Pediastrum 



' Brefeld, Untcrsuchungen aus dem Gesammtgebiete der Mykologie. Heft 6. 



