THE MORPHOLOGICAL COMPOSITION OF PLANTS. 



73 



this inference. In the compound leaf, Fig. 65, the several 

 lateral growths a, b, c, d, are manifestly homologous ; and 

 on comparing a number of such leaves together, it will be 

 seen that one of these lateral growths may assume any de- 

 gree of complexity, according to the degree of its nutrition. 

 Every fern leaf exemplifies the same general truth still bet- 

 ter. "Whether each sub-frond remains an undeveloped wing 

 of the main frond, or whether it organizes itself into a group 

 of frondlets borne by a secondary rib, or whether, going 

 further, as it often does, it gives rise to tertiary ribs, is 

 clearly determined by the supply of materials for growth ; 

 since such higher developments are habitually most marked 

 at points where the nutrition is greatest ; namely, next the 

 stem. But the clearest evidence is afforded among the Alga-. 

 which, not drawing nutriment from roots, have their pa^ts 

 much less mutually dependent ; and are therefore capable of 

 showing more clearly, how any part may remain an append- 

 age or may become the parent of appendages, according to 

 circumstances. In the annexed Fig. 130, 

 representing a branch of Ptilota plumosa, 

 we see how a wing grows into a wing-bear- 

 ing branch, if its nutrition passes a certain 

 point. This form, so strikingly like that of 

 the feathery crystallizations of many inor- 

 ganic substances, proves to us that, as in 

 such crystallizations, the simplicity or com- 

 plexity of structure at any place, depends 

 on the quantity of matter that has to be 

 polarized at that place in a given time.* 



* How the element of time modifies the result, is shown by the familiar fact that 

 crystals rapidly formed are small ; and that they become larger when they are 

 formed more slowly. If the quantity of molecules contained in a solution is rela- 

 tively great, so that the mutual polarities of the molecules crowded together in 

 every place throughout the solution are intense, there arises a crystalline aggre- 

 gation around local axes ; whereas, in proportion as the local action of molecules 

 on one another is rendered less intense by their wider dispersion, they become 



