424 Scientific Proceedings^ Royal Duhlin Society. 



tuber development. The cork-protective mass is very evident, but is not 

 derived from the typical sub-epidermal layer, as a glance at {a) will show. 

 It is a purely protective cork developed over those parts exposed by the 

 splitting of the cortex ; {b] shows us secondary thickening in a bundle. 

 To recapitulate briefly : — 



1. The A group develops a very large quantity, indeed, of secondary wood. 



2. The B group develops a small quantity of secondary wood. 



3. The leaf-cuttings develop a small quantity of secondary wood, but 

 also an elaborate periderm below ground, and most frequently above ground 

 an active local division aud multiplication of many medullary cells. 



4. The D group develops practically no secondary wood, but occasionally 

 a multiplication of medullary cells is to be seen, as in 3. 



Interesting though it be to observe tliese changes in an organ which had 

 reached its normal full differentiation, it is no unique case. That cells in their 

 normal adult condition can become active again, and even like the medullary 

 cells which gave us our interfascicular cambium, and thence traoheides and 

 tracheae, can give rise to tissue which in the normal course of their develop- 

 ment they never form, is a widespread phenomenon. A few examples 

 selected from the literature of the subject will let us realize how widespread 

 it is. Many of the examples will be referred to later in our discussion of the 

 possible causes of the changes in the petiole. 



Thus De Vries (17) mentions a case of the adventitious development of a 

 vegetative bud on the flower-head of Pelargonium zonule in place of a flower 

 bud. Such an appearance is comparatively common. I have one myself in 

 a very small stock of Pelargoniums. De Vi'ies, however, preserved the bud, 

 which he kept growing for three years. The flower-stalk, which is even more 

 transitory than the petiole, developed changes similar to those in our petiole. 



Vochting (18) gives us quite a series in his work on tubers. 1. In one 

 case he planted tubers of Oxalis crassicaulis, so that they were only half 

 buried. In some examples sprouts developed from the over-eartli portion and 

 roots from the underground part. In this way he made the tuber part of 

 the branching system of the plant. He describes tlien the quick and large 

 development of the vascular bundles in the tuber bundles, which normally are 

 exceedingly small. The typical secondary elements — tracheides and fibres — 

 are described by him. 2. Again, the plant Bousdngaultia baselloides normally 

 develops tubers from underground nodes. If, however, an internodal piece 

 be rooted, the lower end swells into a tuberous formation. This occurs in 

 young and old stems indifferently. The swelling in the main is due to divisions 



