THE TISSUES OF THE STEM 57 



from them downwards to the branches, and finally to the cylindrical 

 trunk. It culminates towards the base, where there is the greatest 

 leverage. The butt must stand firm from whatever quarter the wind 

 may blow. It cannot yield as the branches can without loosening its 

 hold on the ground. Naturally it is here that the trunk is thickest. 

 As it tapers upwards it becomes more pliant ; but a condition of 

 successful resistance is the perfect recovery of trunk, branches, and 

 twigs when the wind falls. The mechanical effectiveness of the tissues 

 within the several parts is demonstrated by any tree that stands erect 

 and unbroken after a wind. 



It is a familiar fact that a detached twig soon wilts by evaporation, 

 and more quickly in a wind than in still air. It requires water in 

 order to recover. In the tree exposed to the wind the young shoots 

 normally retain their firmness. During the wind they will have been 

 exposed to more than normal loss by evaporation, but still they are 

 firm, showing that they have been supplied with sufficient water to 

 make good their loss. The trunk, and the distributing agency of its 

 branches and twigs have carried out their water -conducting function. 

 Clearly the water-conducting system meets the requirements. 



The rapid production of leaves and flowers in early summer is in 

 itself evidence of the storage capacity of the trunk and branches. 

 The material required for their formation is gained previously. The 

 rapid development in the spring depends upon its transfer from the 

 storage tissues in the trunk and branches to the buds that were 

 dormant during the winter. Thus the tree demonstrates by results 

 obvious to any observer the efficiency of the tissues of the trunk and 

 branches for mechanical resistance, water-conveyance, and the 

 storage and transfer of materials. Not only this, but also the method 

 of thickening of the stem is such that it meets the. growing demands 

 of the enlarging plant. Finally, the adjustment of the surface tissues 

 to the increasing bulk is peculiarly effective. Not only does the de- 

 velopment of cork, and of that heterogeneous covering of bark, give 

 protection to the surface of the increasing stem, but it provides for 

 the removal of effete tissue. The old phloem, with its cells charged 

 with tannin, crystals of calcium oxalate, and many other substances 

 no longer required, together with old and collapsed sieve-tubes, 

 would be a useless burden to the stem. It is cut off by cork, and shed 

 with the decay of the outer layers of bark. The trunk that shows 

 such features as these is highly organised indeed. It is characteristic 

 of those plants which are recognised as the most advanced, viz. the 

 Seed-Bearing Plants, and particularly the Dicotyledons. 



