PROTECTION 



241 



structure of a vein is practically identical with the struc- 

 ture of a small vascular bundle, which is natural inasmuch 

 as the veins are the extensions of the vascular bundles into 

 the leaves. The larger veins often exceed in thickness the 

 rest of the leaf; they may fill the cross section from epi- 

 dermis to epidermis and cause a swelling in it, but the 

 veinlets are em- 

 bedded in the 

 mesophyll. The 

 larger veins evi- 

 dently serve as a 

 mechanical frame- 

 work for the blade 

 as well as serving 

 as paths of move- 

 ment. 



The veins are seen to be so related to the soft tissues of 

 the leaf that their delivery of raw materials and their 

 absorption of manufactured food is not difficult to under- 

 stand. No transportation system could more thoroughly 

 penetrate the region which it serves than the veins pene- 

 trate the leaves. (See Figure 84.) 



The xylem part of the vein lies above the phloem part, 

 nearer to the palisade tissue. This is the position we would 

 expect in view of the fact that in the stem the xylem lies 

 within the phloem. 



Fig. 84. 



The ending of a xylem strand among cells of 

 the mesophyll. 



58. Protection. — Certain features of leaves not yet 

 considered may be grouped under this general title. The 

 shedding of leaves and the movements of leaves are such 

 features. Protection from excessive transpiration is the 

 principal advantage which the plant derives from both of 



