154 BOTANY. [CHAP. iv. 



make the best of whatever set of surroundings they may 

 encounter, the result being that there are at least ten 

 thousand known species, or about one-tenth of all known 

 flowering plants scattered over the surface of the globe. 



It has been stated that ants and minute insects are 

 as a rule useless in effecting the cross-fertilization of 

 flowers, except in the case of some of the less differen- 

 tiated open kinds; and in the more highly evolved types 

 very varied arrangements are present for the purpose of 

 preventing such creeping insects from reaching the in- 

 florescence. Amongst such contrivances may be noted 

 the presence of a viscid substance on the stem, or the 

 presence of numerous hairs pointing downwards and 

 becoming longer and more numerous in the region of 

 the flowers, as in the cow-parsnip (Heracleum spliondy- 

 lium). In the wild teasel (Dipsacus sylvestris), the 

 opposite pairs of leaves are adherent to each other by 

 their margins for some distance, forming a receptacle 

 capable of holding a considerable quantity of water that 

 surrounds the stem just above the origin of the leaves, 

 and any insect creeping up the stem as far as the first 

 pair of leaves is confronted with this miniature pool, 

 which must be crossed before the stem can again be 

 reached ; and as there are usually from six to eight pairs 

 of leaves, each with its pool of water, it can be under- 

 stood that insects incapable of flight, and at the same 

 time not possessed of natatory powers, rarely reach the 

 inflorescence at the top of the stem. A similar contri- 

 vance is met with in other plants. 



In concluding this portion of the subject allusion must 

 be made to the various modes by which the inflorescence 

 is protected against adverse climatic conditions, also 



