EXTERNAL. CONFIGURATION OF PLANTS. 179 



of a few cells. The oospore from which a j)lant is 

 developed is, however, a spherical body, and, were it 

 to develoj) equally in every direction, must inevit- 

 ably give rise to an adult plant of the same shape. 

 At an early period of development, however, an axis 

 of groivth may be discerned in most plants. We are 

 unable to explain why the growth of the embryo is 

 restricted to particular directions, but the utility of 

 this restriction will become sufficiently apparent as 

 we proceed. The judicious pruning away of certain 

 parts— the arrest of development in one direction- 

 seems to be as essential to advancement of organisa- 

 tion as is the free and full development in other 

 directions. These limitations to growth, so necessary 

 to the specialisation of organs, may possibly be the 

 result of an internal struggle going on among the 

 ]3arts of the same organism. Be this as it ma^', the 

 spherical shape is one singularly disadvantageous to 

 vegetable development. The spherical shape of such 

 plants as EcJunocactus is only an apparent exception 

 to this rule. In this case the form of a sphere has 

 been assumed as a provision to counteract the 

 adverse conditions in which many of the Cacti live. 

 Exposing the least surface to the atmosphere possible 

 in proportion to the mass, these spherical plants 

 must assimilate slowly ; but there is this comj)ensa- 

 tion that the amount of evaporation is reduced 

 to a minimum — a consideration of the utmost 

 importance to plants inhabiting arid countries. 

 From the fact that the amount of evaporation de- 

 pends on the extent of surface exposed to air, the 

 spherical form is the best adapted for retaining 

 water. In this connection we may refer to the 

 typical form of a leaf, which is also very largely 

 determined in relation to the elimination of the 

 water absorbed by the roots. A typical leaf consists 

 of a thin expanded plate or lamina, with or without 

 a leaf-stalk. This shape affords a very large extent 

 of surface; and when the numerous stomata and 

 amount of water daily transpired by leaves are 



