186 PHYSIOLOGICAL CLASSIFICATION OF TISSUES. 



work is secured. These adaptations are as marked in the inter- 

 nal anatomy as in the external configuration. 



517. The parts of a living being which have definite kinds of 

 work to do are known as organs 1 (cf. epyof, work). Since they 



1 The organs of the higher plants are reducible to three members ; that is, 

 three types of structure, which bear to each other definite relations of position 

 and sequence of appearance. These members are the root, stem, and leaf, --to 

 which some add also the plant-hair. In Sachs's Vorlesungen, the number of 

 members is given as two ; namely, root and shoot. 



In their very youngest state all the modified leaves upon a given plant are 

 indistinguishable from each other ; the leaves which are to become petals, 

 stamens, leaf-traps, or tendrils, are like those which are to be ordinary foliage. 

 The same is true of modified stems and modified roots ; however diverse in 

 shape and function the modified stems or branches of a plant may finally be, 

 they are at their very beginning precisely alike. 



In the determination of the rank of an organ, that is, its reference to one of 

 the three plant-members already enumerated, the following criteria are em- 

 ployed : (1) its position with respect to other parts; (2) its nascent condi- 

 tion ; (3) its presence or absence in organisms obviously allied to the one in 

 which it occurs, its rank in these not being obscure. 



So far as the organs seen by the naked eye are concerned, it is seldom that 

 any serious difficulty exists in the application of at least one of these criteria 

 to the determination of their rank, and it is generally possible to use more 

 than one. But it is different in the case of the histological organs, for (1) the 

 position can be made out only in sections of the given part ; (2) their early 

 nascent condition is the simple cell, common to all tissues ; (3) it is not easy 

 to determine whether an organ exists in a rudimentary form in allied organisms 

 or is wholly absent from them. 



It is so difficult to apply these criteria to the study of tissues, and the 

 results obtained are so contradictory, that there is no complete agreement 

 among botanists as to what constitutes a histological member except the sim- 

 ple cell itself. In fact, as stated in 191, it is doubtful whether with the 

 material now at hand it would be possible to construct a satisfactory system 

 of tissue elements or histological organs upon a purely morphological basis. 

 Even in the systems which most nearly approach this there are some physio- 

 logical notions which have affected a few of the minor divisions. 



A classification of tissues upon the basis of physiology alone is open to 

 serious objections ; one kind of work in the plant can be performed by diverse 

 tissues, and on the other hand one kind of tissue can perform more than one 

 kind of work. This is illustrated by the structural elements through which 

 mechanical ends are reached ; the long bast-fibres, woody fibres, collenchyma, 

 and short sclerotic parenchyma, very diverse elements, but accomplishing the 

 same result. Yet one of these, namely, the woody fibres, is among the most 

 important of the elements by which crude liquids are carried through the 

 plant. 



Moreover, *in the examination of the minute structure of a part it is not 

 easy to discriminate between the different offices which one of its given ele- 

 ments may fill, because the element is associated with so many others in the 

 formation of a complex organ. 



