224 GENERAL PHYSIOLOGY 



beset with hairs ; in a drought this stalk is rolled up like a cork- 

 screw into a beautiful spiral (Fig. 88, a), but when moistened it 

 becomes straightened, one turn after another unrolling itself by 

 swelling and extension of the inner side (Fig. 88, &). 



The movements of the so-called elaters on the spores of the 

 horse-tail are very interesting and striking because of their 

 rapidity. The ripe spores of the horse-tail are spherical cells 

 surrounded by a cellulose-wall. This wall is split into two bands 

 the elaters (Fig. 89), which run in a spiral from above downward 

 around the whole ball, being fastened to each other and to the 

 spore itself at a certain spot in the equator. If the spores, 

 slightly moistened, be brought under the microscope, the two 

 bands are seen to lie in two parallel spirals and form a closed 

 capsule about the spore (Fig. 89, a). If they be allowed to dry, 

 the two spirals become extended into straight bands (Fig. 89, &} 



FIG. 89. Spore of a horse-tail, a. The elaters in the moist state are curled around the cell 

 6. The elaters in the dry state are rapidly spread apart, 



through the drying and shortening of their outer sides. If, while 

 observing with the microscope, one breathes upon them in this 

 extended state, they are seen to coil themselves in spirals about 

 the spore with excessive rapidity, their outer surfaces extending 

 by swelling. At the moment when the moisture of the breath 

 disappears, the bands extend again with equal rapidity ; and the 

 experiment can be repeated, like all experiments on swelling, as 

 often as one wishes. 



Swelling-movements are very common among plants, and some 

 of them play an important role in plant life. The great power 

 that can be developed by swelling can be realised from the fact that 

 huge rocks can be split with wedges of swelling wood. 



