276 PHYSIOLOGICAL DEVELOPMENT. 



garden plants, which when held up by tying them to sticks 

 have weaker stems than when they are unpropped, and sink 

 down if their props are taken away. Again, there is the 

 evidence supplied by roots. Though the contrast between 

 the feeble roots of a sheltered tree and the strong roots of 

 an exposed tree, may, like the contrast of their stems, be 

 mainly due to difference of nutrition, and therefore supplies 

 but doubtful evidence, we get tolerably clear evidence where 

 trees growing on inclined rocky surfaces, send into crevices 

 that afford little moisture or nutriment, roots which never 

 theless become thick where they are so directed as to bear 

 great strains. Suspicion thus raised is strengthened 



into conviction by special evidences occurring in the places 

 where they are to be expected. The Cactuses, with their 

 succulent growths that pass into woody growths slowly and 

 irregularly, give us the opportunity of tracing the conditions 

 under which the wood is formed. Good examples occur in the 

 genus Cercus, and especially in forms like C. crenulatus. 

 Here, from a massive vertically-growing rod of fleshy tissue, 

 two inches or more in diameter, there grow at intervals lateral 

 rods similarly bulky, which, quickly curving themselves, take 

 vertical directions. One of these heavy branches puts great 

 strains on its own substance and that of the stem at their 

 point of junction ; and here both of them become brown and 

 hard, while they continue green and succulent all around. 

 Such differentiations may be traced internally before they 

 are visible on the surface. If a joint of an Opuntia be sliced 

 through longitudinally, the greater resistance to the knife 

 all around the narrow neck, indicates there a larger deposit 

 of lignin than elsewhere ; and a section of the tissue placed 

 under the microscope, exhibits at the narrowest part a con 

 centration of the woody and vascular bundles. Clear 

 evidence of another kind has been noted by Mr. Darwin, in the 

 organs of attachment of climbing plants. Speaking of Sola 

 rium jasminoides he says: &quot;When the flexible petiole of a 

 half- or a quarter-grown leaf has clasped any object, in three 



