146 SCIENCE PROGRESS. 



tion, that the vascular bundles are sometimes differentiated 

 in the middle of a homogeneous ground tissue, no trace 

 of a special endodermis or pericycle being visible at any 

 time, we could not predicate the existence, in such cases, 

 of a central cylinder in the stem. And further, if such 

 a condition obtained in the majority of instances (certainly 

 an unlikely supposition) we should not, of course, be 

 justified in predicating the general existence in the stem 

 of a central cylinder, and this would necessitate such a 

 radical modification in the generalised statement of the 

 facts, that the stelar idea would lose the greater part of 

 its significance. We shall have to recur to a discussion 

 of the limit of the cylinder, but these simple considera- 

 tions are insisted upon here, because they are apparently 

 lost sight of in much of the current writing of Van Tieg- 

 hem's adherents. It seems to be implicitly assumed that if 

 a good anatomical distinction can be made in a certain 

 number of cases, it is permissible to generalise the distinc- 

 tion and erect it into a morphological doctrine. The 

 existence of those cases to which the doctrine does not 

 apply is either ignored, or the distinction is said to be 

 " theoretical ". There is of course no such thing as a true 

 "theoretical" distinction which is not also actual. The 

 fallacy arises from a tendency to regard all morphological 

 doctrine as of absolute value, whereas its value is never 

 anything but relative. What we have to decide in any 

 given case is the amount of this relative value, and whether 

 that amount is sufficient to make the doctrine express a 

 general truth so far as the objects under consideration are 

 concerned. 



The foregoing reflections lead us naturally to consider 

 those cases which Van Tieghem himself excepts from the 

 application of the stelar doctrine, namely, the cases of 

 "astely". Already in the earliest paper containing the 

 germ of the stelar idea we find certain cases not covered 

 by the general statement of the existence of a cylinder in 

 the stem. In 1886 these cases together with other similar 

 ones were called astelic, and more recently still the concep- 

 tion has been further elaborated. 



