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, will 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, 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 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, 



