ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 



0G9 



pulling it in the direction of the earth's centre ; as well as of wind, which 

 tends to bend it horizontally. 



Stems must be strong enough to resist these strains, or they will fall 

 to the ground and possibly break in two. 



Botanists have invented the terms positive geotropism " for the 

 result of the action of gravity ("geotropism" only meaning "turning 

 earthwards "), and " apogeotropism " or "negative geotropism" for the 

 growth of the stem in the opposite direction, or upwards. They express 

 facts, but in no way explain how they come about. Gravity has no power 

 to do anything but attract all matter, alive, dead, or inorganic, in a direc- 

 tion perpendicular to ihe surface of the earth, according to well-known laws. 

 Consequently, when a stem grows erect we must look for some cause which 

 induces it to grow i7i opposition to the force of gravity. This was pri- 

 marily light.* Stems were primarily incited {i.e. in their evolutionary 

 history) to grow in the direction of the greatest illumination, viz. the sky ; 

 but in so doing they were obliged to grow in opposition to gravity. This 

 has now become a hereditary feature independent of light. For stems 

 will now grow vertically in total darkness, and if the end of a shoot which 

 has been "drawn" to one side by a lateral source of light be covered 

 over, it will then proceed to grow vertically upwards. 



In so doing the stem must support its own weight. As long as 

 plants are quite young " the rigidity of succulent shoots, especially in 

 length, depends essentially upon the [elastic and turgid] conditions of the 

 layers of tissue." t They soon, however, develop wood and other tissues 

 of support. 



Roots. — As soon as the radicles have developed roots their force of 

 growth becomes enormous, increasing of course with age. Thus Sir J. D. 

 Hooker writes in his "Primer of Botany "t : — " With such force does growth 

 go on that stones of walls are frequently displaced by roots. In tropical 

 countries the destruction of buildings is often caused by the power of 

 growing roots; and neither conquering nations, nor earthquakes, nor fires, 

 nor tempests, nor rain, nor all put together have destroyed so many works 

 of man as have the roots of plants, which have all insidiously begun their 

 work as slender fibres." 



Trees may often be seen with a considerable portion of their roots out 

 of the soil. This has resulted from their growth against the resistance of 

 the soil, acting as a fulcrum, so that they have elevated the trunk of the 

 tree, which now appears supported, as it were, on slanting props diving 

 into the ground. 



Stems. — Soon after germination, the stem develops special'" supportive 

 or mechanical tissues," such as wood-fibre, bast-fibre, sclerencbyma, col- 

 lenchyma, &c.§ 



* For further explanation and illustration the reader is referred to Tlie Origin of 

 Plant Structures, p. 197, fT. 



t Sachs, Physiolofjy of Plants, p. 217. X P. 38. 



§ Wood-fibre consists of thickened spindle-shaped cells which, compacted together, 

 make wood. BaU or liber-fibre consists of very long thickened cylindrical cells taper- 

 ing at the ends, much thickened, but remaining elastic. Sclerenchynui in stems 

 (flower- stalks, Ac.) consists of elongated cells thickened with very hard matter. 

 The " grit, " of pears and the tissue of " stones " of fruits are cells similarly hardened. 

 Collencliyma is a tliickening of the angles and sides of a softer substance, giving a 

 more coriaceous or leathery texture to the organ. 



