136 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



xvi, pp. 525 and 386). This, however, becomes distended, and breaks up in the 

 adult into a number of meristeles scattered through the massive parenchyma. 

 In Marattia and Angiopteris their number and polycychc arrangement form 

 a very marked feature. No effective endodermis is found dehmiting these 

 meristeks of the adult stem from the surrounding tissue. It thus appears 

 that while both OpJiioglossum and the Marattiaceae have a definite and in- 

 tegral stele in the young stem, the adult shows a disintegration and a loss of 

 delimitation of the vascular tissues, which are carried to a higher degree than 

 in any of the other primitive Ferns. Moreover, the sappy stock is traversed 

 throughout by a continuous system of intercellular spaces. The appearance 

 is as though the stelar construction had entirely broken down ; but the 

 ontogeny shows that a normal stelar structure underlies the ruins. (See 

 West, Ann. of Bot. 19 17, p. 361.) 



Fig. 128. Ophtoglossiini Bergiannni, Schlecht. /4 = transverse section of the stock, showing a large 

 semilunar stele, with wide foliar gap from which a small leaf-trace- strand is passing out. Z? = another 

 section showing the overlapping of foliar gaps. ( x 200.) No endodermis is to be seen. 



The physiological interpretation of this seemingly anomalous state, and 

 of the structural facts which mark the advance from the primitive pro- 

 tostelic condition to a high degree of stelar disintegration, must be held 

 over to Chapter VIII. Here it must suffice to have recognised the successive 

 steps which appear to have led to that state, whether traced by comparison 

 of adult structure or by following the ontogenetic history. 



Secondary Thickening 



Lastly, the innovation of cambial activity remains to be mentioned. 



Though this has played so important a part in the Vegetable Kingdom as 



a whole, in the Filicales it never established a real hold. It appears early 



in the young plant of various species oi Botrychium, for instance in B.Lunaria, 



