CHAPTER X 



SIZE A FACTOR IN STELAR MORPHOLOGY^ 



The vascular system of the shoot in the FiHcales has always commanded 

 attention because of its high complexity. It will be shown, as the phyletic 

 treatment of the Ferns is developed, how greatly the varying details which 

 have been described in general terms in the foregoing Chapters may be made 

 use of in their classification according to Descent. But this is only one 

 aspect of the interest which their structure arouses. The question remains 

 why such unusual vascular systems should have been developed at all. 

 When the condition of Ferns as growing organisms is considered, and the 

 limitation which their peculiar structure sets upon the performance of their 

 functions, it will appear that increase in Size, carried out under certain 

 structural restrictions, has been a decisive factor in leading to their extra- 

 ordinary vascular development. According to the Principle of Similar 

 Structures, so long as the same form and material are maintained, the surface 

 and the resistant strength vary as the square of the linear dimensions, and 

 the bulk, and consequently the weight, as the cube. It follows from this that 

 what may be possible for a small structure may be quite impossible for a 

 large one. The mechanical application of the principle is already familiar 

 with reference to the animal body. For instance, the columnar legs of the 

 elephant are held to be the inevitable sequel to its large size and consequent 

 weight, while the thin arched legs of insects are only possible where the 

 body itself is small and light. Botanists have, however, been slower in 

 applying the principle to the study of plants. It is true that the question 

 of the practicable limit of size of trees has been discussed from this point 

 of view, and it is recognised that a change either of material or of method 

 of construction would be necessary for effective growth beyond the limits 

 already reached by some of them. But the principle is also applicable to 

 other points of construction, such as the size and constitution of individual 

 cells, and even to the forms of chloroplasts ; as well as to various problems 

 of distribution of tissue-ma.sses, and to the physiological functions which 

 they perform. For in many cases these depend upon the proportion which 

 the surface bears to the bulk of the organ. 



1 Chapter x is based upon an Address on " Size, a neglected factor in Stelar Morphology," 

 which was delivered at the Anniversary Meeting of the Royal Society of Edinburgh, Oct. 25, 1920, 

 and published in the Society's Proceedings, Vol. xli, p. i. In that Address the subject was treated 

 generally, for Vascular Plants at large. Here a special application has been made of the Principle 

 of Similar Structures in elucidating the peculiarly complex Vascular System of the Filicales. 



