THE MORPHOLOGICAL COMPOSITION OF PLANTS. /! 



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 

 stein. But the clearest evidence is afforded among the Al&amp;lt;ja\ 

 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 /J &quot; 

 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 bccunw 



