GROWTH AND MOVEMENT 451 



an idea that agrees with what is known of the intensity of light required 

 for photosynthesis (see p. 371). Yet the arrangement is seldom as regu- 

 lar or complete as it is sometimes described, and effective protection 

 from light is secured mainly in other ways. Aside from their own 

 amoeboid movements the chloroplasts are subject to displacement by 

 movements of the protoplast, as in streaming (below) . 



The nucleus also changes its position in the cell " spontaneously " 

 or in response to certain stimuli, notably to wounding. Nothing is 

 known as to the significance or mechanism of such movements. 



Streaming. — In very many active cells a streaming movement of 

 portions of the protoplasm has been observed. The layer closest to 

 the wall does not participate in the movement, and though the chloro- 

 plasts, when any are present, are not necessarily involved, they are often 

 swept along when they lie deeper. The rate of the motion varies with 

 temperature and with other conditions that affect the general activity of 

 the protoplasm, and the movement may be entirely stopped by appro- 

 priate stimuli. Nothing is known as to the causes or the effects of 

 these movements, though they are extremely common and perhaps 

 universal. The idea that they facilitate the more rapid distribution of 

 foods and solutes in the cells and so hasten osmotic transfer of materials 

 would be more plausible were streaming less common and vigorous in 

 those cells, e.^. in hairs, where such a process seems of slight importance. 



In some diatoms the protoplasm partly protrudes through a longitudinal median 

 slit (the raphe) in the valves, and streaming movements in this outer belt, reacting 

 against the water or the substratum, propel the cell slowly in the direction opposite 

 to the outer streaming. The counter-stream, of course, moves within the cell wall. 



Surging movements of the protoplasm in the coenocytic hyphae of Mucor and 

 other fungi have been seen, but their causation and significance are unknown. 



6. TURGOR MOVEMENTS 



Motor organs. — In a considerable number of plants thin-walled turgid 

 cells are so arranged that the position of the organ of which they form 

 a part depends upon the relative turgor of these cells. In most cases the 

 organs are leaves, either foliage or flower leaves, and the structure is such 

 that the motor organ curves only in one plane, the distal part rising or 

 falling with the variations of turgor. Examples of these motor organs 

 are afforded by the leaves and leaflets of the Leguminosae and the 

 Oxalidaceae, by the stamens of Berberis, and by the stigmas of Mimidus; 



