88 STUDIES IN EVOLUTION 



a minute fraction of the length. The Climbing Palm, or 

 Ratan, has stems of great length and tenuity. It has been 

 stated that stems two hundred metres long have been ob- 

 served having a uniform thickness of only from two to four 

 centimetres.^^ The diameter of such a stem would be only 

 one or two ten-thousandths of its length. The length of 

 the internodes is another conspicuous character in climbing 

 plants, and both this and the slenderness of the stems suggest 

 the results obtained by growing ordinary plants in the dark, 

 where the conditions are adverse to increased vitality. 



The transfer of function from one part of the plant to 

 another, usually by a process of retrogression or degradation, 

 is also very common. The first growth above the ground is 

 a leafy stalk. Later, after the plant has attained a consider- 

 able height, the lower portion puts out quantities of rootlets 

 and loses its foliage. The rootlets may be mere dry threads 

 or points of support for the stem; or, if they happen to 

 encounter a crevice containing soil, they develop into true 

 absorbent organs. In others the ends of the growing stems 

 or any point on the stems, upon reaching the earth, may 

 put out vigorous roots. These facts seem to show a lack of 

 positive differentiation throughout the plant, which admits 

 of the substitution of a lower structure for a higher by the 

 suppression of a higher function. 



Lastly, the general spininess of climbing plants and bram- 

 bles is a well-known and conspicuous character. Kerner^^ 

 says that "most, if not all, plants which weave into the 

 thicket of other plants are equipped with barbed spines, 

 prickles and bristles." These spiniform processes seem to 

 fall naturally into two classes: first, those produced by the 

 suppression of stipules, leaves, petioles, branches, etc. ; and 

 second, those appearing as simple eruj)tions on the surface. 



The suppression of normal plant organs into special struc- 

 tures, as tendrils and claspers, is extremely common, and, as 

 already shown, this process, if carried far enough without 

 complete suppression, will favor the production of a spini- 

 form growth representing the axial elements of the organs. 



