ELEMENTARY VITAL PHENOMENA 239 



noticed, in thick pseudopodia the former plainly upon the peri- 

 phery, the latter in the axis of the strand. According as the former 

 or the latter preponderates, the pseudopodium gradually extends or 

 shortens itself. If the two are equally strong, it remains extended, 

 its length not changing. The phenomena of contraction and 

 expansion may be studied very easily and fully in the long, filose 

 pseudopodia of Foraminifera, such as Orlitolites (Fig. 98). The 

 phase of expansion, i.e., extension, consists always in a centrifugal 

 flow of the living substance into the surrounding medium, the phase 

 of contraction, i.e., retraction, in a centripetal flow from the 

 periphery to the central cell-body. Expansion is characterised by 

 an increase of surface, contraction by an effort toward a spherical 

 form. 



Protoplasmic streaming in plant-cells follows the same plan. A 

 cell from a stamen-hair of Tradescantia mrginica is a cylindrical, 

 closed cellulose-capsule (Fig. 99, A), in which the protoplasmic cell- 

 body with its nucleus is enclosed. The protoplasm forms upon the 

 inner wall a continuous, extremely delicate layer, the so-called 

 primordial utricle, from which there extend in various directions 

 through the lumen of the capsule, filled with cell-sap, protoplasmic 

 strands which anastomose with one another and at one point lodge 

 the nucleus. Both in these long, protoplasmic strands and in the 

 primordial utricle a continual, protoplasmic streaming is visible, 

 which corresponds perfectly to the protoplasmic streaming in the 

 pseudopodia of Rhizopoda. When the protoplasm in the various 

 strands flows in an inco-ordinated, irregular direction, the movement 

 is termed by the botanists circulation ; when it follows continually 

 one definite direction, rotation. This phenomenon would corres- 

 pond, therefore, to the protoplasmic movement of a rhizopod-cell, 

 such as Orlitolites, in an undisturbed state, in which the protoplasm 

 in elongated pseudopodia streams continually both in a centrifugal 

 and a centripetal direction, i.e., in which the phases of contraction 

 and of expansion are equally developed. In the plant -cell such a 

 complex system of currents has arisen by division of the mass of 

 protoplasm into single, anastomosing strands, so that the distinction 

 between centrifugal and centripetal currents no longer holds ; the 

 same is true, also, of large Rhizopoda, such as the plasmodia of 

 Myxomycetes, the whole body of which resolves itself into a richly- 

 branched, pseudopodial network. The phase of contraction may, 

 however, be brought out here very clearly and easily by stimuli. 

 As in the Rhizopoda, it is characterised by the protoplasm 

 becoming contracted into globules (Fig. 99, B), which flow together 

 and under certain circumstances finally form a large lumpy mass 

 about the nucleus. This is the complete analogue of the phase of 

 contraction in the Rhizopoda, where the pseudopodia retract them- 

 selves so that the body assumes a more or less spherical form. 

 Hence, the phenomena of protoplasmic streaming are, in principle, 



