CHAPTER IX. — MODE OF LIFE OF THE MYCETOZOA. 449 



The requisites for germination are the same in sclerotia and cysts as in 

 spores, as was stated above on page 428, where also will be found all that is known 

 of the external causes which lead to the formation of these states. 



Section CXXVII. Some account has necessarily been already given in section 

 CXIX of the phenomena attending the life of the Plasmodia. For many 

 general questions which here come under consideration, the reader is referred, in ac- 

 cordance with the purpose of this book, to works on general physiology, and especially 

 to Pfeffer's Physiology, vol. II, chap. 8 and Stahl's latest publication on the subject, 

 and the account here given must be confined to a short review of their mode of life. 

 This has been investigated chiefly in the plasmodia of the Physareae, Fuligo especially, 

 which are readily procured. What is known of other forms appears to agree 

 with the accounts given of the Physareae, but requires more exact investigation. 



Movement of plasmodia. The internal causes of the changes of shape, of 

 the protrusion and withdrawal of processes and the interior streaming of granules, 

 which are attendant on the organisation of the protoplasmic body and are 

 to a great extent unknown to us, cannot of course be discussed in this place. 



The most important external causes of the movements of the plasmodium and 

 of the changes in its form are, the illumination, the distribution and movement of the 

 water in the substratum, the chemical nature of the environment, and the conditions of 

 temperature. It is uncertain to what extent purely mechanical influences are also 

 operative. RosanofT's former assumption of geotropic movements has proved to be 

 without foundation. We may therefore, in accordance more or less with the general 

 terminology of the movements of growth, speak of phenomena of heliotropism, 

 hydrotropism and rheotropism, tropholropism and thermotropism. 



Heliotropism. A vegetating plasmodium stretches out its branches and 

 reticulations uniformly in every direction on uniformly moistened surfaces, such as 

 paper steeped in water and kept in a dark or equably but faintly illuminated chamber. 

 If the intensity of the illumination is increased, the power of movement, according to 

 Baranetzki, is generally diminished, and if the amount of illumination is different in 

 different portions of the expanded surface, the branches are drawn in from the bright 

 side and others are put forth towards the darker side ; the plasmodium also moves 

 towards the darker side. The direction of these movements is independent of the 

 direction of the beams of light which fall on the plasmodium, being determined only 

 by the intensity of the illumination. 



Hydrotropism. If while all other conditions are uniformly favourable the 

 water is unequally distributed in the substratum, the vegetating plasmodia, if not on 

 the point of forming spores, withdraw from the drier spots when the dryness has 

 reached a certain degree and wander towards the moister. 



Rheotropism. If a stream of water is made to flow slowly through a 

 moistened porous substratum, such as filtering paper or strips of linen cloth, the 

 plasmodia which are vegetating on the moist surface wander in the direction of 

 the stream, without regard to the particular direction in space in which it moves. 



Trophotropism. Vegetating plasmodia spread out on surfaces which yield 

 little or no nutriment move towards bodies which contain nutrient substances as 



[4] Gg 



