THE STELA R THEORY. 135 



is to be devoted. The results so obtained are to be 

 applied, in a further series of memoirs, to the solution 

 of a number of morphological problems, such as the true 

 nature of tendrils, tubers, spines, phylloclades, ovules, etc., 

 and finally, to the elucidation of the laws of symmetry 

 governing the structure and relations of the ideal colony 

 that would be formed if every seed germinated in situ. 



This elaborate scheme for "un cercle d'etudes an- 

 atomiques complets et fermes " enables us to understand 

 the strength and the weakness of the author's stelar theory. 

 The imperative desire to reduce the anatomy of vascular 

 plants to a perfect system depending upon simple laws of 

 symmetry governing the arrangement of the vascular tissue, 

 has been the means of giving us a doctrine, luminous indeed, 

 and of wide significance, but scarcely of that rigidly uni- 

 versal application which its author claims. But here again, 

 as is so often the case in the history of science, the attempt 

 to work out logically the various implications of such a 

 theory, has been of the utmost value in clearing our ideas 

 and extending knowledge, not only by stimulating to the 

 discovery of new facts, but by forcing us to examine the 

 foundation of our conceptions. 



Of Van Tieghem's scheme, as it stood in 1870, how- 

 ever, only the first memoir, that on the root, was ever 

 written. The author demonstrates the fundamental identity 

 of structure in the roots of all vascular plants, and obtains his 

 anatomical definition based on the symmetry of the vascular 

 system. He shows that the vascular tissue of a young root 

 forms a central cylinder which contains near its periphery 

 " faisceaux liberiens" (phloems) alternating with "faisceaux 

 vasculaires" (xylems) united by "cellules conjonctives ". 

 Hence the vascular system is symmetrical in relation to a 

 line, which is the organic axis of the organ. The stem 

 agrees with the root in this last point, but on the other 

 hand has its "faisceaux libero-vasculaires," " reunis directe- 

 ment par le parenchyme primordial ". Where the main 

 root passes into the main stem there occurs a "cessation du 

 tissu conjonctif special, qui se trouve remplace par le 

 parenchyme primitif ". This sentence is specially interest- 



