XER0PI1YTE ASSOCIATIONS. 



205 



ravages of animals (see §105). Examine Figs. 176, 177, 

 178, 179, 180, 181. 



153. Anatomical adaptations. — It is in connection with 

 the xerophytes that some of the most striking anatomical 

 adaptations have been 

 developed. In such 

 conditions the epider- 

 mis is apt to be cov- 

 ered by layers of 

 cuticle, which are de- 

 veloped by the walls 

 of the epidermal cells, 

 and being constantly 

 formed beneath, the 

 cuticle may become 

 very thick. This 

 forms a very efficient 

 protective covering, 

 and has a tendency to 

 diminish the loss of 

 water (see §35). It is 

 also to be observed 

 that among xerophytes 

 there is a strong de- 

 velopment of palisade 

 tissue. The working 

 cells of the leaves next 

 to the exposed surface 

 are elongated, and are 

 directed endwise to 



the surface. In this way only the ends of the elongated 

 cells are exposed, and as such cells stand very closely to- 

 gether, there is no drying air between them. In some 

 cases there may be more than one of these palisade rows 

 (see §32). It has been observed that the chloroplasts in 

 these palisade cells are able to assume various positions in 



Fig. 177. Two plants of common gorse or furze 

 (Ulex), showing the effect of environment : b 

 is a plant grown in moist conditions ; a is a 

 plant grown in dry conditions, the leaves and 

 branches having been almost entirely developed 

 as thorns.— After Lothelier. 



