444 



PHYSIOLOGY 



stimulation the total growth is much greater than it is under uniform 

 conditions. 



5. LOCOMOTION AND STREAMING 



Locomotion limited. ■ — Locomotion is restricted among plants to the 

 simplest forms (with a few exceptions to those that are unicellular), 

 and to the gametes, especially the male gametes, of the multicellular 

 plants. The reason for this is doubtless to be found in the restriction of 

 freedom to move imposed by the cell wall — in effect a sort of strait- 

 jacket — in which the protoplast incases itself. Even when the proto- 

 plast moves, as it often does, within this case, 

 its movements do not bear against the outer 

 medium and therefore do not propel it about. 

 The only exception to this restriction occurs 

 in those plants whose wall is perforate; then 

 the protoplasm protrudes through the opening 

 so as to operate against the outer medium, or 

 in a few cases it excretes mucilage forcibly 

 against the medium or the substratum and so 

 pushes itself slowly along. 



Rate. — When the protoplast changes its 



shape suddenly, quick swimming and darting 



Fig. 677.— Sperm of A?^e- movements result; when slowly, the move- 



phrodium, with flagella. ment is perceptible only because magnified by 



— After Yamanodchi. , . t .1 -r 



the microscope. In the very swiftest move- 

 ments the absolute translation is small, say 50 mm. per minute; and 

 in the sperms of ferns, which under the microscope seem to be going 

 fast, the rate is only o.i to 0.2 mm., per minute. Measured relatively, 

 as in terms of size, and taking account of the resistance of the medium, 

 the translation is seen to be very rapid. The very fast human runners 

 cover about 50 times their own length (100 yards) in 10 seconds ; the 

 swarm spores of Ulva can travel 100 times their own length in the 

 same time; and the spiral sperms of a fern {N ephrodium) can do 50 to 

 100 times their length (as coiled) in 10 seconds (fig. 677). 



Amoeboid movements. — The slow movements are a kind of creeping, 

 and are of two sorts, amoeboid and excretory. Amoeboid movements 

 (so called because characteristic of Amoeba, a genus of infusoria) are 

 found rarely among plants, being known only in the plasmodia of My.xo- 

 mycetes, a group of organisms with so many animal characters that they 



