DIVISION OF LABOR IN THE PLANT. 187 



are parts of a whole, the organism, they must have definite 

 relations to each other as regards position and office. 



518. The relations of origin and position, so far as the organs 

 of the plant are concerned, are discussed in the first volume ; 

 the relations of origin and position of the component parts of 

 their structure have occupied the earlier portion of the present 

 volume. From a review of the facts there presented, it appears 

 that any given part may subserve different ends ; for instance, a 

 leaf may carry on its proper work, namely, that of assimilation, 

 and at the same time may aid as a tendril, and, in the case of 

 Nepenthes, as a stomach for digestion. On the other hand, it 

 is equally clear that the same kind of work may frequently be 

 performed by different parts. For instance, the proper work of 

 the leaf can be carried on by any green tissue ; not merely in 

 proper leaves, but in the cortex of young stems, and even in the 

 outer tissues of young roots of certain aerial plants. It is there- 

 fore sometimes advantageous in Vegetable Physiolog3 r to distin- 

 guish between systems of tissues having different offices, rather 

 than between organs which are often masses of heterogeneous 

 tissues. 



519. Among the systems of classifications of tissues chiefly 

 upon a physiological basis is that of Haberlandt, which is as 

 follows : 



A. The Protective System. 



1. Of the surface (Epidermis, cork, and bark). 



2. Of the skeleton (Bast-fibres, libriform cells, collenchyma, 



and sclerotic parenchyma). 



B. The Nutritive System. 



1. Absorbing system (Epithelium of roots and the root- 



hairs ; absorbing tissue of haustoria, etc.). 



2. Assimilating system (Chlorophyll parenchyma, both pali- 



sade and spongy). 



3. Conducting system (Conducting parenchyma, vascular 



bundles, latex cells and tubes). 



4. Storing system (Reserve-tissues of seeds, bulbs, and 



tubers ; water- tissue, etc.). 



5. Aerating system (Aeriferous intercellular spaces, together 



with their external openings, stomata and lenticels). 



6. Receptacles for secretions and excretions (Glands, oil, 



resin, and mucus canals, crystal-sacs, etc.). 



To these might be added the groups of tissues concerned in 

 reproduction. 



