THE DISl'KRSIOX OK SEKDS AND SI'ORES. 123 



which gasteropods gnaw by means of their rasping 

 strap-shaped tongue makes it doubtful if they 

 swallow entire seeds, and their slow restricted 

 movements prevent their being very efficient dis- 

 persers of adhesive seeds. On the whole, we should 

 perhaps rather look for provisions by which seeds 

 are protected against snails. Very possibly the 

 tubercles on the perianth of the dock and on the 

 carpels of Reseda may be intended to divert the 

 attention of snails and caterpillars from the seeds. 

 A spinous or prickly surface presents an insurmount- 

 able barrier to the ascent of a soft-bodied animal ; 

 but the leaves, stems, and flower-stalks of Reseda 

 and Rumex are smooth, so that a snail can easily 

 reach their fruits. For this reason the seeds may 

 require some protection ; and it may well be that 

 the succulent tubercle affords the necessary pro- 

 tection by serving as a sop to satisfy the intruders 

 without encroaching on the more highly cherished 

 seeds. 



10. Hum.an Agency. — Besides the timber, grain, 

 and vegetables which he cultivates, man uninten- 

 tionally aids in the dispersion of numerous plants. 

 Every great wave of migration has doubtless been 

 accompanied by a host of vegetable camp-followers. 

 Many common weeds have followed Europeans to 

 the New World, the Cape, and the Australian colo- 

 nies. Among these emigrants are the common shep- 

 herd's-purse, groundsel, chickweeds, docks, thistles, 

 rye-grass, bai'ley-grass, and many others. Foreign 

 species are constantly being introduced with impor- 

 tations of grain, timber, wool, etc. There are well- 

 known instances of foreign seeds having come to 

 this country with the straw used to pack wine-cases. 

 Quite a number of plants have been introduced in 

 ships' ballast; and on the banks of streams, where 

 imported wool is washed, strange plants are from 

 time to time observed to have established themselves. 

 Many escapes from cultivation also appear to have 

 become naturalised with us. As examples of the 



