42 



PLANT PHYSIOLOGY AND ECOLOGY 



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The xylem usually occurs in narrow strands, which are united in 

 the middle. The number of xylem strands is usually two or four, 

 but it varies for different species. The phloem occurs in a corre- 

 sponding number of plates or masses, alternating with the xylem 

 strands, and lying near or next to the pericycle. The mesenchym 

 gives rise to the major portion of the cambium ring necessary for 

 secondary growth, and sometimes produces a pith. 



55, Origin and structure of root-hairs. The particular function 

 of the epidermal layer is absorption. The cortical region has no 



special function, though diffusion is confined 

 to it very largely. The vascular region 

 serves as a pathway for the transport of 

 water. The function of mechanical support, 

 which is peculiar to the fibrovascular bun- 

 dles, is entirely secondary in importance, 

 owing to the support afforded by the soil. 

 The epidermal layer of the roots of water 

 plants shows practically no differentiation 

 with respect to absorption. Practically all 

 cells of the surface, except those that are 

 very young or very old, absorb water with 

 equal readiness. The roots of all land plants, 

 except of those that grow in very wet places, 

 are especially adapted to the absorption of 

 water from the soil by means of root-hairs. 

 The latter are not separate cells, but thread- 

 like prolongations of the epidermal cells 

 from which they arise. They have thin un- 

 modified walls lined with a scarcelv demon- 

 """vhiL'mSrw strable layer of protoplasm. Root-hairs do 

 sica alba, grown in not arise over the whole surface, but are 

 ZoitS'K^aZ 0"fi"l to ^ particular region behind the 

 and their relation to tip which consists of meristem. As the root 

 (AftS'sIchsf ''''''^' elongates, the older hairs die off as new ones 



are formed, and the zone of root-hairs main- 

 tains an almost constant width. Not all of the epidermal cells 

 produce root-hairs, two or more unmodified cells standing between 

 adjacent hairs. These unmodified cells are doubtless able to absorb 

 water, but they can not be very active under ordinary conditions, 

 since the root-hairs obtain most of the water available. Behind 



