THE UNIT IN FLANT'LIFE. 543 



less fusible quality than the bitumens of France and Switzerland, and 

 has not been produced in the compressed form. It is extensively made 

 into mastic, the fabrication of which already amounts to ten thousand 

 tons a year, and is rapidly increasing. La Nature. 



-- 



THE UNIT m PLAKT-LIFE. 



By BYRON D. HALSTED, Sc. D. 



MAN, being himself distinctly individualized, endeavors to find 

 the unit of existence in all other forms of life. He meets with 

 no great difficulties among the higher animals, but is perplexed and 

 sometimes discouraged when search is made for the individual in the 

 lower animals and in plants. 



A child is an easily-recognized unit of life at its birth, and is no 

 more than a single individual when it has reached mature middle life, 

 or the decline and decay of old age. A limb may have been lost on 

 the field of battle, or an eye removed by a surgeon, but there is no re- 

 placement of the lost parts. The human individual may suffer divi- 

 sion, but it is a mutilation, and not a multiplication of the living unit. 



In some respects the seed of a plant is analogous to the young of 

 the higher animals ; it is the result of a sexual union and the starting- 

 point of a continuation of the species. For these and other reasons 

 the seed may be, and has been, called the unit of life in the higher 

 plants. But what possibilities are contained within the coats of a 

 single seed when the proper conditions for its growth and propagation 

 are secured ! When we look beyond the dry and inactive seed, which 

 can be held upon the tip of the little finger, and note what it may 

 produce ; when we know that such a single seed has been the starting- 

 point of a variety of fruit that now has its representatives as full- 

 grown trees in thousands of orchards all through this broad land, we 

 must either expand our idea of a plant-unit until it is too great and 

 comprehensive to be of service, or seek some other basis of individu- 

 ality than that which is in some respects analogous to the accepted one 

 among the higher animals. The idea of the identity of the individual 

 among the more complex and perfect forms of existence in the two 

 kingdoms of life may be dismissed, because the methods of propaga- 

 tion in the two are far from the same. 



If we take some common plant of the higher orders any tree or 

 shrub, or even a herb it will be found, upon careful study of its 

 structure, that there is an almost monotonous repetition of parts. It 

 will also be observed that these parts may be grouped under three 

 heads, to which the common names of root, stem, and leaf are applied. 

 The root includes that portion of the plant, whether aerial or subter- 



