76 MORPHOLOGICAL DEVELOPMENT. 



hence the development of those self-detached cells which 

 characterize some plants. Conversely, physiological units 

 which form a small group involved in a larger group, and are 

 subject to all the forces of the larger group, will become sub 

 ordinate in their structural arrangements to the larger group 

 will be co-ordinated into a part of the major whole, in 

 stead of co-ordinating themselves into a minor whole. This 

 antithesis will be clearly understood on remembering how, 

 on the one hand, a small detached part of a hydra soon 

 moulds itself into the shape of an entire hydra; and how, 

 on the other hand, the cellular mass that buds out in place 

 of a lobster s lost claw, gradually assumes the form of a claw 

 has its parts so moulded as to complete the structure of 

 the organism: a result which we cannot but ascribe to the 

 forces which the rest of the organism exerts upon it. Con 

 sequently, among plants, we may expect that whether any 

 portion of protoplasm moulds itself into the typical form 

 around an axis of its own, or is moulded into a part subor 

 dinate to another axis, w r ill depend on the relative mass of 

 its physiological units the accumulation of them that has 

 taken place before the assumption of any structural arrange 

 ment. A few illustrations will make clear the validity of 

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

 lateral growths a, &, c 9 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 bear 

 ing frondlets, is determined by the supply of materials for 

 growth; since such higher developments are most marked 

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

 stem. But the clearest evidence is afforded among the Algce. 



