A MANUAL OF BOTANY 91 



f). What enemies, if any, do tliese burs have ? (You 

 may hnd the fruits with the larva' of insects in them.) 



7. How long does it take each bur i)lant to mature from 

 the seed ? 



8. Draw each bur, natural size, and the hooks or struc- 

 tures that cling to animals. 



The downs. 1. Examine carefully a thistle or other 

 downy structure. Is it the whole fruit or the single seed 

 that bears the down ? 



2. Observe carefully the number and size of the hairy 

 structures that nuike the down, also whether they are 

 straight or wavy. 



o. Observe in particular where the down is attached 

 to the seed. Why is the attachment generally above the 

 seed ? 



4. Float a specimen in the air and observe the rapidity 

 of " settling down " to the floor. Blow on the down. Does 

 this action hasten or retard the process of sinking ? How 

 would the wind act on the down ? What connection do 

 you see between these observed facts and the general dis- 

 tribution of thistles and dandelions ? 



5. Kecall, if possible, the " cotton " seen on June days, 

 where Cottonwood trees grow in the neighborhood. ^lake 

 a list of as many downy fruits as you are acquainted 

 with. 



G. Examine the down with (Ij^) and observe the features. 

 Explain, if you iind anything worthy of record. 



7. Make drawings of complete fruits or seeds, natural 

 size ; also of one or more hairs, (m) or (Ij)). 



8. Describe clearly and accurately each down studied. 



9. By reading, discover if any downs are of value to 

 man. 



10. Explain, if you can, the results found in the following 

 " living problem " taken from the " prickly lettuce," a 

 ^' downy " weed introduced from Europe. 



