STELAR THEORIES 



By THOMAS G. HILL, A.R.C.S., F.L.S. 

 Lecturer on Biology, St. Thomas's Hospital 



Ten years ago an article from the pen of Mr. A. G. Tansley, 

 on the then current ideas regarding the nature of the vascular 

 structures of plants, appeared in Science Progress, and, 

 inasmuch as during the last decade a large amount of work, 

 directly bearing on the theory of the stele, has been published, 

 no apology is needed for this present consideration. 



It may at once be stated that what follows, necessarily brief 

 and incomplete, is not intended for the botanist with special 

 knowledge of the subject ; rather, it is designed for those who 

 require an introduction to a somewhat complicated chapter in 

 modern botany. 



If a transverse section of the stem of the sunflower be 

 examined, there will be found a number of vascular strands 

 arranged in a zone, and, if the stem be sufficiently young, separate 

 one from the other. These strands, or bundles, are embedded 

 in parenchyma which, on account of the disposition of the 

 vascular bundles, is divided into two parts, the central medulla, 

 or pith, and the peripheral cortex. 



Immediately bounding the ring of fibro-vascular strands, on 

 its outer side, the presence of a bundle-sheath, or endodermis, 

 may be demonstrated. 



Inasmuch as this brief account is purely descriptive, it may 

 be taken as illustrative of the views held by botanists prior to 

 1886. In those days there was no attempt made to differentiate 

 the constituent tissues of plants into morphological parts, after 

 the manner obtaining in external morphology ; the internal 

 structures were treated rather from the physiological point of 

 view. Thus, no essential difference was supposed to exist 

 between the cortex and pith, the bundles were considered as 

 separate structures, and the endodermis was regarded as being 

 nothing more than a bundle-sheath ; that is to say, it was not 

 thought to have any special virtue in the delimitation of one 

 tissue from another. 



325 



