ECCENTRIC GROWTH. 267 



IV. Eccentric Growth. 



The special type of symmetry observed in a plant body which 

 presents a distinct upper and under surface, differing in appear- 

 ance and physiological function, has been very generally described 

 by the term dorsiventral (Sachs). This term, borrowed from the 

 usage of Animal Morphology, in which this type of symmetry is 

 the rule rather than, as in plants, the exception, naturally implies 

 the possession of a dorsum and a venter, or at least a hypothetical 

 plane dividing the body into two regions which will ultimately 

 become these surfaces. Even in Zoology the convention is 

 admittedly based on the body structure of higher animals, and 

 from these is carried down to less differentiated types, as implying 

 a certain hilaterality of structure. In its application to Botany, 

 therefore, in the absence of anything which corresponds to either 

 dorsum or venter, which appear to be regions ultimately correlated 

 with the evolution of a locomotile body seeking solid food, the 

 term must remain a purely metaphorical expression ; and, as in 

 the case of all borrowed terms, care must be taken lest the 

 suggested analogy be ultimately accepted as an actual fact. The 

 term dorsiventral apparently presented itself to Sachs in prefer- 

 ence to the simpler term bilateral, in that it contained the sugges- 

 tion that the two surfaces must be dissimilar ; and the wider term 

 bilateral would, according to him, include the case, for example, of 

 an erect shoot bearing members in two rows, or with what has 

 been previously termed a symmetrical (1 + 1) construction. For 

 the same reason a diarch root would be bilateral, though few 

 would nowadays object to a diarch root being still classed as only 

 a special case of radially symmetrical root construction. In 



