26() VEGETABLE ORGANOGRAPHY. 



appears to be the more certain way of distinguishing 

 them, especially in Monocotyledons. 



From the prickles replacing hairs, and spines replacing 

 all the other organs, it follows, consequently, that the 

 former are superficial, and the latter intimately connected 

 with the tissue. 



It is remarked that most prickles of stems or petioles 

 are curved with the point directed downwards, as in the 

 Rose ; but this rule ought not to be taken as a general 

 one, for there exist perfectly straight prickles in several 

 Mimosea3. 



Thorns, whatever be their origin, are generally the 

 defensive weapons of certain plants, serving to protect 

 them from the teeth of large animals, &c. Some, 

 perhaps, may serve either to penetrate the earth to 

 favour the natural sowing of seeds, as in Trifolium sub- 

 terraneum; or to hook the fruits or seeds of certain 

 plants to the wool of animals, in order to transport them 

 to a distance, as in the Burdock. 



The existence, and consequently the use of spines, is a 

 fact entirely peculiar to certain species, sometimes to cer- 

 tain varieties, and is but very slightly connected with the 

 general symmetry, and consequently with the funda- 

 mental laws of their organization. We frequently observe 

 in the same families and the same genera, species, some of 

 which are furnished with, and others devoid of thorns. 



Let us observe, on concluding this chapter (and this 

 remark will be equally applicable to the following), 

 that the analogy which is observed in the manner in 

 which the sepals, petals, stamens, and carpels can, like 

 true leaves, change into spines or tendrils, tends to 

 confirm the identity of origin of these organs. 



