EXERCISE 55 

 WHAT ADVANTAGE IN POSSESSING BURS OR STICKERS? 



Materials. Cockleburs and beggar-ticks or Spanish needle ; any 

 other kinds of " stickers " ; spring balance graduated in grams. 



Directions for work. 1. Oocklebur. Put one of the burs against 

 your sleeve and notice how readily it takes hold. Pull it off, 

 noting how strongly it holds on. By what means does it hold 

 so firmly ? Examine the projections with which the surface of 

 the bur is thickly studded. Are they simple straight spines, 

 like pins, as they at first seem ? What peculiarity of their 

 shape enables them to hold ? To measure the strength of one 

 of these spines, attach a thread or light cord to the spring 

 balance and tie a loop in the free end of the thread. Hook one 

 of the spines into this loop and pull on the spring balance until 

 the spine breaks or lets go of the thread. What was the maxi- 

 mum amount registered by the balance before the spine failed? 

 Repeat with several other spines. What is the average strength 

 with which the spines are able to hold fast? 



2. Beggar-ticks. Thrust the points of the beggar-tick into 

 the cloth of a garment, noting the relative amount of force 

 needed to insert the points and to withdraw them. Do they 

 seem to hold effectively ? Examine the spines of an uninjured 

 beggar-tick with a magnifier to find out what peculiarity of 

 structure enables it to hold fast. It is sometimes noted that 

 after beggar- ticks have been adhering to the clothing for some 

 time the points come through on the inside, though they appar- 

 ently did npt do so at first. Can you find anything in their 

 structure that would favor a gradually deeper penetration ? 



3. Comparison with other burs. Make a rapid study of other 

 burs which are available, noting resemblance to the two types 

 studied above and apparent effectiveness of their equipment. 



4. Comparison with other seeds. Burs commonly become en- 

 tangled in the coats of grazing animals in the autumn. How 

 long would they probably remain attached to the animal? In 

 the case of domestic animals, how far might it reasonably be 

 supposed that they might be carried in the interval ? How far 



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