FTLICALES 491 



of the protoxylem are seen in longitudinal section to be spiral or reticulate, 

 as in other Vascular Plants (Fig. 378, p.xv). 



The sieve-tubes are also spindle-shaped, and are without companion-cells. 

 Their cellulose walls are swollen. Where two sieve-tubes adjoin, numerous 



thinner sieve-areas of irregular outline are borne. They arc found to be 

 perforated by very fine protoplasmic threads extending between highly 

 refractive globules that adhere to the walls (Fig. 380). Such tracheides and 

 sieve-tubes are characteristic of Ferns, and with differences of detail, they 

 are present in other Pteridophytes as well. 



The anatomy of the leaf in Ferns resembles that of Seed-Plants down even 

 to the collateral structure of the vascular strands. Being chiefly shade- 

 loving plants chlorophyll is usually present in the cells of their epidermis, 

 and the differentiation of the mesophyll into palisade and spongy parenchyma 



Fig. 381. 

 Transverse section of part of pinnule of Dryopteris ( x 150), showing epidermis, 

 and the spongy mesophyll, with an internal glandular cell. 



is not marked (Fig. 381). In these respects they resemble the leaves of Angio- 

 sperms of similar habit. In the roots of Ferns, as in those of Seed-Plants, there 

 is a superficial piliferous layer, a broad cortex, and a contracted stele. But 

 usually the inner cortex is very strongly lignified, up to the endodermis, which 

 is thin-walled (Fig. 382). The pericycle which follows is variable, sometimes 

 being greatly enlarged as a water-storage- tissue. The protoxylems are 

 peripheral, and two or sometimes more in number, the phloem-groups alter- 

 nating with them. In fact the root of a Fern is constructed essentially on the 

 plan of that in Seed-Plants. As there is no secondary thickening the roots of 

 Ferns are all fibrous. The lateral roots arise opposite to the protoxylems, 

 and there they originate from definite cells of the endodermis, which may often 

 be recognised beforehand by their size and content-. 



While we recognise the substantial similarity of Ferns and Seed-Plants in 

 respect of form and structure of stem, leaf, and root, these plants dilter in tin- 

 construction of their apical meristetns. In Seed-Plants these are small-celled 

 tissues, and more or less definitely stratified (pp. 17, 89). In Ferns such 

 as Osmunda, Dryopteris or Polypodium, a single large cell, the apical or 

 initial cell, occupies the tip of each growing part. It has a definite 



