1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 327 
the surface with which it has come into contact. As light evidently 
directs the locomotion of the spore, it is reasonable to suppose that 
it also determines its polarity on germination. It is hardly probable 
that light influences the growth of the holdfast; nor has gravity any 
effect, for the spores attach themselves in various positions and may 
even be suspended from the surface of the water. Chemical stimulus 
can hardly have any part in the matter, except perhaps in relation to 
the bacteria. The growth of the holdfast would appear, then, to 
result from contact with a solid body, as an irritable response to contact 
stimulus. This matter we wish to report upon at length. 
The influence of contact upon growing plants and their parts has 
been studied from various points of view by many investigators and 
upon a great variety of plants. It is now clear that contact with a 
solid object may influence growth in three ways: first, as to its 
direction, as shown by sensitive tendrils (16); second, as to its rate, 
also as shown by tendrils (5); and third, as to its kind, as shown 
by the formation of haustoria in dodder (14). Contact similarly 
influences the direction, rate, and kind of growth in germinating 
zoospores. 
If it is true that contact influences the direction, rate, and kind 
of growth in germinating zoospores, the character of the surface with 
which the contact is effected should be reflected in the germinated 
zoospores, a rough surface producing a different and more pronounced 
effect than a smooth surface. To test this hypothesis, we used a 
Yanlety of substances for the purpose of catching the zoospores, viz. 
Wisps of cotton, clean cover glasses, glass roughened by corrosion 
and deposit, strips of gelatine, freshly split mica, and the surface of 
the water itself, These we interposed between the swarming spores 
= the light. The spores came to rest and attached themselves 
within eighteen hours. 
Spores germinating on the surface of the water in contact with 
Particles, diatoms, and masses of bacteria developed long slender 
Processes, sometimes forked, but always hypha-like-in appearance 
(figs. 5, 6). Others on the surface, and in contact with no solid matter 
whatever of visible amount, formed the most rudimentary hold-fasts, 
° -like Processes of very small size (figs. 1, 2, 3, 4). Those on 
gelatine, the acid reaction of which had been duly neutralized, formed 
dust 
