PART II. METAMORPHOSIS 



LECTURE XX 



INTRODUCTION 



IN the discussion of metabolism mention has frequently been made of the 

 work done by the organism as a result of respiratory and other analogous 

 katabolic processes. These activities were also touched on in Part I, for it 

 must be clearly understood that the movements of raw food material, plasta, 

 and products of assimilation are just as much expressions of such activities as 

 the movements of an entire organ, as, for example, the assumption of the erect 

 position by a stem laid horizontally. Having now considered chemical physio- 

 logy we must turn our attention more especially to the activities of the 

 organism, but in doing so it should be noted that we are changing not the subject- 

 matter but the point of view, since chemical changes are never absent when the 

 shape or position of a plant or plant-organ is altered. Change in material, in 

 energy, or in form, occur simultaneously in nature, and it is only for con- 

 venience of investigation or exposition that we may legitimately institute sub- 

 divisions in our science. Two such divisions, generally recognized, are 

 metabolism and transformation of energy ; we venture to add a third, viz. 

 change of form. Before attempting to justify the institution of this section it 

 may be well to inquire on what the characteristic form of an organism, and 

 especially of a plant, really depends, and also what manner of alterations it 

 undergoes. 



Let us approach the subject inductively and consider a few examples. 

 First of all take the Myxomycetes, which we have already learned (Lecture I) 

 to regard as naked protoplasmic masses, or plasmodia. Plasmodia are soft, 

 slimy bodies which form irregular networks over their substrata, i. e. decaying 

 leaves, dead branches, &c., and in which continual changes in outline may be 

 observed with the naked eye, simultaneously with changes in position. These 

 movements are even more apparent under the microscope, and one may observe 

 a complete alteration in shape in the course of a few minutes. Phenomena such 

 as these are not, however, what we mean by ' change of form ', and should not 

 be included in the present section of our studies. Just as a viscous liquid spreads 

 irregularly over its substratum, so the plasmodium has no definite shape ; only 

 when it assumes such a shape does it come under consideration in this relation. 

 Under certain conditions, however, the protoplasm of a Myxomycete aggregates, 

 takes on a rounded form, secretes a firm external layer, and its contents divide 

 into a large number of spherical cells. Each of these is capable under suitable 

 external conditions of again becoming a naked mass of protoplasm and of 

 creeping about over the substratum. The slime fungi may thus exist in two 

 conditions, different in shape, in the vital phenomena they exhibit and in their 

 significance. In the 'formless' state it requires a certain amount of water, 

 inorganic and organic nutrients, and a certain temperature, and given these 

 conditions the food-materials are assimilated and the plasmodium grows. 



