178 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



senting the external surface of the body, the inner 

 one the digestive system, with the XDerivisceral 

 space between. An ahmentary system is thus a 

 contrivance whereby the superficial area of an 

 organic body may be extended without any addition 

 being made to its mass. This means of extending 

 the absorbing surface is not available in the case of 

 plants, since their nutrition depends on the free 

 access of light, which would be unable to illuminate 

 an internal cylinder. In the j)itchers of Nepenthes 

 and other carnivorous plants we have an exceptional 

 form which corresjDonds with their excej)tional mode 

 of i)rocuring nourishment. Two animals which out- 

 wardly resemble each other very closely may yet 

 have very different habits and subsist on totally 

 distinct kinds of food. In such cases an examination 

 of their alimentary systems is needed to reveal the 

 differences which separate them. But the case is 

 different in plants, whose organs of nutrition are 

 external. The outward configuration of the plant- 

 body is therefore of greater significance. For the 

 same reason the extent of external surface is usually 

 greater in proportion to the mass of the body than 

 in animals — for example, the area of leaf-surface 

 alone in an elm has been computed to be ^ye acres. 

 No animal ever existed with an external surface at 

 all to be compared with this. This great superficial 

 area is a necessity arising out of the mode of 

 assimilation that obtains in vegetable organisms. 



In our endeavour to trace the relationships subsist- 

 ing between tlie contour of plants and their environ- 

 ment it will be convenient to begin with some 

 common types. 



1. The Spherical Form. — The chief representatives 

 of this form in the vegetable world are of small 

 size — of microscopic dimensions, in fact. The 

 spherical shape is indeed incompatible with great 

 mass, because it affords the least possible area of 

 absorption in proportion to the mass. Spherical 

 plants accordingly consist mostly of a single cell, or 



