(iKRMfXATlOX 263 



\'i)iin_i,f plantlct by clccoinposinL,^ the carbon dioxide 

 of the air and foriniiiL; starch, whilst in contrast to 

 this wc learnt that the seed-leaves of the bean 

 were storehouses onl\-. We are now sufficiently 

 acquainted with the functions of the seed to be 

 able to appreciate the \ariations of the tcs/a, or 

 seed-coat. In numerous instances the spines, 

 prickles, hairs, and other growths on the surface 

 have, in addition to their use in dispersin<,f the 

 seed, an essential pur[>ose in holdinL,^ the seed in 

 its rightful position. We will take cress as our 

 next example, since it ma>' be regarded as a type 

 of all smooth seeds. Cress seed remains intact 

 until water comes in contact with it ; then it 

 becomes slimy b\' the liberation of a mucilaginous 

 cement from the (juter coat layer ; this is, of 

 course, highl)' adhesive, and thus the seeds are 

 fixed firml\' into the soil. 



Another example is that of the little epiphyte 

 (mentioned in our first chapter;, Tillmidsia usnoidcs, 

 or old man's beard. When the seeds leave the 

 capsule they are furnished with silk)' hairs, which 

 enable the lin\- little structures to float through the 

 air ; the)' soon come in contact with the bark of 



