CHANGES OP SHAPE OTHERWISE CAUSED. 179 



traits of structure. In Fig. 256, a shoot of Phyllocactus 

 crenatus, we have an interesting example of a variation 

 essentially of the same nature, little as it appears to be so. 

 For each of the lateral indentations is here the seat of an 

 axillary bud; and these we see are separated by internodes 

 which, becoming broader as they become longer, and narrower 

 as they become shorter, produce changes of form that corre- 

 spond with changes in the luxuriance of growth. 



To complete the statement it must be added that these 

 variations of nutrition often determine the development or 

 non-development of lateral axes; and by so doing cause still 

 more marked structural differences. The Fox-glove may be 

 named as a plant which illustrates this truth.* 



§ 240. From the morphological differentiations caused by 

 unlikenesses of nutrition felt by the whole plant, we pass 

 now to those which are thus caused in some of its parts and 

 not in others. Among such are the contrasts between 

 flowering axes, and the axes that bear leaves only. It has 

 already been shown in § 78, that the belief expressed by 

 Wolff in a direct connexion between fructification and innu- 

 trition, is justified inductively by many facts of many kinds. 

 Deductively too, in § 79, we saw reason to conclude that such 

 a relation would be established by survival of the fittest; 

 seeing that it would profit a species for its members to begin 

 sending off migrating germs from the ends of those axes 

 which innutrition prevented from further agamogenetic mul- 

 tiplication. Once more, when considering the nature of the 

 phaenogamic axis, we found support for this belief in the fact 



* Natural selection may have operated in establishing a constitutional 

 tendency to other sudden abridgments. Mr. Tansley alleges that this is a 

 part-cause of the varying distribution of leaves. He says : — " I have myself 

 made some observations on the length of internodes in the Beech, and am 

 satisfied that it follows quite other laws, connected with the suitable dis- 

 position of the leaves on the branch. Although I have not had the oppor- 

 tunity of following up this line of work so as in any way to generalize the 

 results, I suspect that ' indirect equilibration ' is a widespread cause of such 

 variation." 



