THE STELAR THEORY. 223 



as Van Tieghem does, to support the view that the genus 

 is really astelic. This argument depends on the assump- 

 tion that these walls always separate stelar from extra- 

 stelar tissue, which is not a fact, according to Van 

 Tieghem himself (10, p. 774), and further, a similar line of 

 reasoning would tend to show that the stems of a great 

 many dicotyledons, namely, those in which the pith is the 

 first tissue to be marked off, are also astelic. 



SUMMARY OF RESULTS. 



We have attempted in the foregoing pages to ex- 

 hibit, as clearly as possible, the bearing of well ascer- 

 tained facts of anatomy and development upon the stelar 

 theory as developed by Van Tieghem and his pupils. We 

 may appropriately conclude with an attempt to summarise 

 the results to which we are thus led. 



We recognise in the central cylinder of the axes of the 

 great majority of the higher plants an anatomical region of 

 the first rank to be co-ordinated with the other great 

 anatomical regions, the cortex and the epidermis. The 

 central cylinder consists of vascular tissue (xylem and 

 phloem) and conjunctive tissue (typically parenchyma). 

 In the bulky typical * cylinder the vascular tissue is separ- 

 able into distinct strands corresponding with its centres (or 

 rather lines) of development, and giving to the cylinder a 

 radial symmetry ; the conjunctive of such a cylinder is 

 separable into distinct regions. Typically, also, the inner- 

 most layer of cortex, which abuts on the cylinder is dis- 

 tinguished by special characters. 



Reduced central cylinders are found in various stem 

 structures, especially the thin stems of water plants. The 

 reduction acts first on the conjunctive, which may (though 

 rarely) quite disappear. This leads to the coalescence of 

 the strands of vascular tissue into a more or less solid 

 cylinder. Such a reduced cylinder is always sharply marked 

 of! from the cortex. 



On the other hand we have stems in which it is im- 

 possible to separate the conjunctive from the adjacent 



1 In Sach's sense of " most highly developed ". 



