THE ORIGIN OF FLORAL APPENDAGES. 137 



asserting that nature lias produced them in order to keep ants 

 off ; for that line of reasoning is pretty sure to land us in 

 faulty teleological methods. Wkat causes them is not at 

 present known in all cases ; though we may perceive that 

 certain conditions, as growth in water, can bring about their 

 disappearance, as Dr. Kerner remarked in the case of 

 Polygonum anipMbium, which only has them when growling 

 on land. 



If, however, we ask, for example, Avhy the Sweet-briar has 

 them all over it, and why the Dog-rose has none, I do not 

 know how to reply to the question as yet. We may notice 

 certain coincidences, that hairy herbaceous plants are com- 

 moner in dry situations and smooth ones in watery ; just as 

 root-hairs occur in a loose sandy soil and their absence is 

 noticeable in a heavy one; but we do not know how these 

 di-fferent media actually bring about these changes, though 

 wx' may feel assured that it is solely due to the environment. 



If we, thus, look elsewhere than in flowers for any 

 analogous processes they are by no means w-anting. For 

 example, it is simply the mechanical irritation brought about 

 by contact with a foreign 

 body, probably aided by 

 moisture and a lessened de- 

 gree of light, that causes 

 the epidermal cells of the 

 aerial roots of the Ivy and 

 Orchids (Fig. 42) to elon- 

 gate into adhesive or clasp- 

 ing hairs, so as to grasp the ^ig. 42.-Adhesive epidermal ceUs of roots of 



body for support. This is jancztwsS/''"^' ' ''' ^"''^''"■'■''"^*'' ^''^*^'' 

 only a form of the ordinary 



root-hairs which are immediately developed w^hen the tip is in 

 contact with a moist soil, and each hair grips and glues itself 



