igo6| On the Structure of Roots. 19 



a large persistent parenchyma in which nutritive matters are 

 "stored." They often become tuberous by the excessive develop- 

 ment of this parenchyma, and these are well known from the 

 Orchidecc : Orchis, Spiranthes, Platanthera, etc., also from 

 Hemerocallis, Aconitum, Delphinium, etc. 



The structure of such roots offers really a number of interest- 

 ing modifications, which are very little known so far, and it would 

 be an excellent study to undertake the investigation of their struc- 

 tures, instead of confining ourselves to the other organs alone. It 

 is not, however, an easy matter to study such roots, but by be- 

 ginning with the more simple types, for instance the annual 

 among the nutritive, the various tissues may be readily perceived 

 and distinguished. The most difficult ones are the tuberous 

 storage- roots, and these must always be studied at the various 

 stages of their growth and during several seasons. There are, 

 also, certain types which are called anomalous, as for instance 

 the beet, which is quite difficult to understand, unless the succes- 

 sive stages have been observed. 



With the object ol giving some examples of different root- 

 structures we may begin with an ordinary, annual nuiritive root 

 of Streptopus roseus, of which we have drawn part of a section on 

 Plate I. In this drawing the central cylinder is complete, but the 

 cortex and epidermis is only shown in part. The structure is as 

 follows : 



The epidermis (Ep. ) consists of a single layer and many of 

 the cells are extended into root-hairs (Rh.); beneath this tissue is 

 another single layer, the cells of which are quite thickwalled, and 

 this is the so-called exodermis (Ex.). The cell- walls are more or 

 less suberized, thus the membranes are almost impermeable to 

 water and render thereby an important protection to the interior 

 tissues. In many cases the exodermis possesses, also, the power 

 of contractility, which may be seen from tangential sections, 

 where the radial cell-walls show foldings or undulations, which 

 continue in the longitudinal direction of the root, resulting in con- 

 traction. 



Inside the exodermis follows a parenchyma of several layers, 

 the cortex (C. ); it is in this tissue that nutritive matters are stored 

 in storage-roots. The cells are often loosely connected, thus we 



