526 THE POPULAR SCIENCE MONTHLY. 



fats. It is not always easy to determine of what other uses dis- 

 agreeable or toxic chemical constituents may be to the plant ; the 

 point that concerns us in this discussion is, that they protect it 

 from being eaten, and of this there can be hardly any doubt. 



M. StahPs study of the mechanical defenses of plants is no less 

 interesting than that of their chemical armor. Many of the weap- 

 ons of this character are obvious and well known ; but some of 

 them are more difficult of discovery, while a great variety prevails 

 among them. In the large majority of cases the mechanical 

 defense consists of a hardening of some parts of the plants, which 

 may be general, so as to form a kind of carapace, or local, in the 

 production of hard special organs, such as hairs, thorns, or nee- 

 dles, making it harder for animals to reach the plants. Sometimes 

 the mechanical weapons are associated with chemical qualities, as 

 in the nettle, crane's-bill, Primula sinensis, blessed thistle, etc. 

 They either serve to prevent or impede the access of snails and 

 slugs, to make it harder for them to take hold of the alimentary 

 part, or to cause pain during the eating. 



Hairy plants certainly offer more obstacles to snails going 

 about on them than do glabrous plants. If we place a snail upon 

 a comfrey-plant, it will find itself very uncomfortable, unable to 

 get any hold on the leaves, and continually brought to a stop by 

 the disagreeable contact of the hairs with its tentacles ; and a 

 free snail or slug will be hard to find on this plant. Other hairy 

 plants possess immunity in less marked degrees ; and M. Stahl's 

 conclusions from his experiments as a whole are that chemical 

 armor is more efficient than hairs. In some cases downy plants 

 were preferred, while chemically armored species were always 

 respected. So, when glabrous and downy species of the same family 

 were tested, downy ones were eaten, while smooth ones were left 

 alone. Hence, the hairs afford only an inefficient defense. M. 

 Stahl accounts for this by supposing that, while the smooth plants 

 are protected by disagreeable chemical constituents, the hairy 

 plants are without this armor, or else present attractive qualities 

 of odor or taste, against which their hairs are only an imperfect 

 set-off. 



Some plants are defended by the calcification of their super- 

 ficial cells. The snails would not eat the leaves of Erysimum 

 cheirantoides (treacle-mustard) when fresh, or even when treated 

 with alcohol, but attacked them readily after the carbonate of 

 lime had been dissolved out by acetic acid. The same was ob- 

 served with other plants having a similar property. The grasses 

 are protected against attacks from many animals by the silicifica- 

 tion of the walls of their cells, without which the new enemies 

 that would be added to supplement the assaults of their present 

 foes would make an end of the whole family. This may be tested 



