280 



PLANT MORPHOLOGY 



showing an amphicribral arrangement. Sclerenchyma is usually prom- 

 inently developed. There is no secondary thickening, even in the tree 



ferns. 



Except in protostelic stems, prominent leaf gaps are formed in connec- 

 tion with the departure of loaf traces, as in all the orders of Filicinae. In 

 addition to loaf saps, perforations not related to the departure of leaf 



Fig. 235. Cross section of the central portion of the rhizome of Gleichenia costaricensis, a 

 protostele, X50. 



traces are sometimes developed in the vascular cylinder, especially in 

 ferns with elongated rhizomes. In many forms accessory vascular 

 strands are present, usually inside the vascular cylinder. In the common 

 bracken (Pteridium aquilinum) we have a well-known example of the 

 occurrence of medullary strands combined with a considerable amount of 

 perforation of the vascular cylinder (Fig. 238). Accessory vascular 

 strands are present also in many tree ferns. 



Protostele. The protostele, representing the most primitive vascular 

 type, is found only in a few genera, such as Lygodium, Gleichenia, Hymeno- 

 phyllum, and Trichomanes; but it occurs as the earliest developmental 

 stage in most other true ferns. 



Gleichenia displays a typical protostele (Fig. 235). No pith is present, 



