CHAPTER XX 



THE ANGIOSPERMAE : ROOTS 



There are several reasons for beginning the study of Angiosperm structure 

 with the root rather than with the stem. 



The anatomy of roots is in many respects simpler than that of stems and 

 it is considerably more uniform, perhaps as the result of a rather uniform 

 enV'ironment underground, which is in marked contrast with the extreme 

 variability of the conditions affecting aerial shoots. Root anatomy is also 

 more ancient in type than that of stems. Roots preserve, in their primary 

 condition, the actinostelic arrangement of the vascular tissues, which in the 

 lower Pteridophyta is characteristic of both stems and roots. Thus, while 

 the structure of the stem has undergone profound changes in the course of 

 evolution from the level of the Lycopods upwards, the root has altered 

 relatively little and consequently in the Angiosperms its structure contrasts 

 strikingly with that of the stem, which is not the case in the Lycopods. 



Furthermore, the relationship of structure to function, the division of 

 functions between tissues and the development of anatomical organization 

 are all matters which are \&r\ clearly illustrated in roots. 



We may here suggest as a guiding principle in all such considerations 

 as these, that we should look upon structure as arising out of function, not the 

 reverse. A function may exist before there is any anatomical structure 

 particularly associated with it, and it may, in fact, exist along with many 

 other functions in a unicellular organism. The history of structural evolution 

 has been, to a great degree, the history of the way in which conditions of 

 life have induced the localization of functions in the body, with consequent 

 effects on each particular tissue concerned, which we can trace in the observed 

 differentiations of structure. 



General Characteristics. 



Roots are distinguishable by a complex of characters, and while we should 

 avoid any such idealistic concept as that of a " typical " root, we may say 

 that the organs classed as roots are those which show all, or at least a majority, 

 of the following characters : — 



1 . Absence of leaves and hence, of course, absence of buds. 



2. Possession of a root cap over the apex, 



3. Endogenous origin and branching. 



4. Phloem and xylem on different radii in the primar}' structure. 



5. A relatively short zone of growth at the apex. 



6. Possession of root hairs near the apex. 

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