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 Ne- movements result; when slowly, the move- 
phrodium, with fiagella. me nt is perceptible only because magnified by 
After YAMANOUCHI. ... to 
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 (Nephrodium) 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 Myxo- 
mycetes, a group of organisms with so many animal characters that they 
