October, 1928 



EVOLUTION 



Face Eleven 



The Amateur Scientist 



A Monthly Feature conducted by Allan Strong Broms 



HOW SEEDS TRAVEL 



A walk through the fields or woods these 

 autumn days gets one all stuck up with 

 seed burrs of Spanish needh\ beggar ticks, 

 cocklebur or burdock. All have seeds with 



Bursting pod of wild bean. Hool^s on 



fruits of coclilebur and burdocl;. 

 Winged seeds of maple. 



grapple-hooks to catch the furry coats of 

 wandering mammals and hook free rides to 

 distant spots so the plants may spread 

 widely. Another method, more pleasing to 

 the animals, is employed by the plants that 

 enclose their seeds in tempting, pulpy fruit 

 which ripens in conspicuous and attractive 

 colors so that the seeds may be swallowed 

 and carried away. As the seeds are hard 

 and indigestible, they are presently dropped 

 and thus planted far away from the parent 

 plant. 



For a big problem of plant life is to get 

 the young out from under the stifling 

 shadow of the old, a problem serious with 

 us humans, too. Wide dispersal also means 

 larger possible numbers with the conse- 

 quent greater chances for race survival in 

 the struggle for existence. Every favorable 

 spot can be planted and occupied, so local 

 extinction may not mean race extinction. 



In the West, another effective method of 

 seed dispersal is well known in the "tumble- 

 weeds". After growing into large balls of 

 twigs full of seed pods, they break loose 

 and roll off before the wind, shaking out 

 their seeds along the way. Usually they pile 

 up in great heaps against the fences until 

 the wind turns and sends them tumbling 

 over the prairie again. The wind also serves 

 many plants that grow wings on tlieir 

 seeds, the maple, basswood, Bignonia and 

 Ailanthus for instance. Many smaller plants 

 grow plumes and tails and tufis for seed 

 parachutes, such as Clematis, fireweed. 

 bulrush and dandelion. 



Water currents of streams or ocein 

 serve to spread many seeds tliat are buoy- 

 ant or become so when swelled from snak- 

 ing. Seeds may be carried over the seas a 

 thousand miles in this manner, if only the 

 ocean currents be right. Seeds buried in 

 mud may stick to the feet of a migrating 

 bird and be carried even farther. Tliat 

 such mud is loaded with seeds was proven 

 by Darwin in one of his patient experi- 

 ments. From a tea-cup of mud, kept cov- 



ered for six months, he pulled 537 plants 

 that germinated from the seeds it con- 

 tained. 



Several kinds of plants contrive to throw 

 their seeds for some distance without de- 

 pending on outside agencies. The fruit of 

 a wild cucumber fills with water until it 

 bursts, squirting a stream and its seeds 

 several feet. The pods of the violet and 

 witch hazel are so constructed that the 

 ripened seeds are pinched and shot out 

 with considerable force. The pods of the 

 wild bean, "touch-me-not" and a domesti- 

 cated "artillery plant" develop a twisting 

 tension which causes them to throw their 

 seed away violently when they suddenly 

 break. Tropical travellers often hear the 

 detonations of the exploding seed vessels of 

 Hura crepitans, the "monkey dinner bell." 



This is the season for wild seed planting. 

 Next spring and summer, when you are 

 down on your knees weeding the garden, 

 you will know how well the job has been 

 done. 



THE WHEELED ANIMALCULE 



At the American Museum of Natural 

 History a new group has recently been 

 completed, modeled in glass and wax. mag- 

 nify ng a bit of pond water and its life a 

 million times in volume. The most numer- 

 ous of the inhabitants shown are the minute 

 animals known as rotifers, whence the 

 name "The Rotifer Group" given to this 

 exhibit. The rotifers are a numerous tribe 

 of many species, seldom longer than one- 

 thirtieth of an inch. They are found ahnrist 

 everywhere in protected waters, either fresh 

 or salt, and are partly responsible for keep- 

 ing such waters pure and clean by eating 

 the fine decaying particles which would 

 otherwise accumulate and befoul. They are 

 broadly classified into the limnetic or open 

 water group and the littoral group found 

 among the stems of water plants. 



Viewed through a low-powered micro- 

 scope, they are seen to be tube shaped, a 

 tail "foot" at one end and at the other a 

 broad head with what looks like a pair of 

 moving wheels, whence the popular name, 

 the "wheeled animalcule." These wheels 

 are really rows of fine, vibrating hairs or 

 cilia used in feeding and swimming. If the 

 foot remains attached (by means of cement 

 glands), the vibrations of the cilia cause 

 water currents winch sweep food particles 

 into the centrally located mouth. When the 

 foot releases, however, the cilia row the 

 animal straight forward quite swiftly. As 

 the body is transparent, the internal organs 

 and processes can be seen. There is a com- 

 plete digestive system and a simple nervous 

 system with a head ganglion or "brain". 

 It is one of the most simply organized of 

 the multicellular animals and probably not 

 far from the direct line of our primitive 

 ancestors. 



Its senses are limited as its possible re- 

 sponses are limited. There may be one, 

 two or more eye-spots and some feelers for 



touch and perhaps smell. The slightest dis- 

 turbance, because it means danger, causes 

 the rotifer to retract or swim away. It feeds 

 mostly on smaller unicellular plants and 

 animals, such as bacteria, algae and proto- 

 zoans. Its simple, yet hardened, "jaws" 

 grind this food before the swallowing. In 

 some species, the mouth parts are modified 

 to pierce plant stems and suck out their 

 contents. In predatory species, the "jaws" 

 are pincers or are otherwise adapted for 

 carnivorous pursuits, even to eating micro- 

 scopic crustaceans. 



The habits vary with the many species. 

 The ordinary rotifers have a protective 

 skin, but a few build hard tubes out of 

 round ''bricks" made by the animals them- 

 selves out of mucus. Some attach them- 

 selves to and are carried around by larger 

 animals. Still others live quietly in colo- 

 nies. At least one species creeps into the 

 globular body of the protozoan cell colony 

 Volvox, feeding parasitically upon the nour- 

 ishment of its host. 



ADAUiliiy of a typic&i rotifer [FUxtrotrocha ptlromtiam) 



Anatomy of typical Eotifer. Side anfl 

 top views. Corona of vibrating cilia. 

 Mastax or jaws. Foot cement glands for 

 anchoring. 



The individuals we usually see are fe- 

 males, the males being smaller, fewer and 

 shorter lived. The female can ordinarily 

 do without the male, for the unfertilized 

 eggs will produce young. But when winter 

 approaches, many small eggs are laid and 

 from them come males in great numbers. 

 The eggs they fertilize are of a tougher 

 breed, able to withstand the winter cold. 

 Should the water dry away, the animal also 

 dries into a speck of dust that the wind can 

 carry off. In this condition, the rotifer (or 

 its eggs) can survive for years. James 

 Murray of the Shackleton Antarctic Expe- 

 dition sank an eighteen-foot shaft to the 

 bottom of a completely frozen lake, thawed 

 out the mud and got living rotifers, frozen 

 in perhaps centuries before. Because of 

 this tenacity to life, rotifers can be gath- 

 ered any time, wet, dry or in the egg, and 

 a colony cultivated indoors for winter 

 study. 



The American Museum of Natural His- 

 tory, New York City, publislies two book- 

 lets, excellently written and illustrated, 

 dealing with the rotifers and the new Roti- 

 fer Group. They can be secured from the 

 museum at ten cents each. 



