CHAPTER VI 



CELLULAR CONSTRUCTION 



Since the growing points of stem, leaf and root are capable of continued 

 growth and multiplication of cells, they are the ultimate source from which 

 the tissues composing those parts arise. Following the analogy of animal 

 embryology it might reasonably be expected that the characters of the meri- 

 stems would be specially important for elucidating the origin of parts and 

 the morphology of tissues. But it was long ago shown by De Bary {Conip. 

 Anat. Engl. Edn. p. 23) that the analogy is misleading, and more recent ob- 

 servations have confirmed the view that apical segmentation is not a secure 

 guide in the morphology of mature tissues. Nevertheless, though the facts 

 may not bear that direct interpretation which was at one time expected from 

 them, and though the apical cell or cells and their segments cannot be held 

 as dominating the genesis of tissues, the comparison of meristems in the 

 Filicales has another, and a distinct importance. It is the object of this 

 Chapter to show how the facts of segmentation may lead to a sedation of 

 Ferns, which can be used systematically. It already forms, in point of fact, 

 the foundation for their phyletic grouping: for it is on segmentation that the 

 generally accepted distinction of Leptosporangiate from Eusporangiate Ferns 

 is based. 



The Plant-body of the Pteridophytes is of cellular construction so de- 

 veloped that all the tissues originate by cell-lineage from superficial cells. 

 There is no dermatogen present at the apical points, giving rise to a super- 

 ficial skin covering the tissues of distinct origin within. At the growing tips 

 superficial cells give rise by anticlinal and periclinal segmentations on the 

 one hand to superficial cells, on the other to the deeper-lying tissues. Thus 

 in not being stratified the apices of Pteridophytes differ from those of the 

 Higher Flowering Plants, in which the apical regions are more or less stratified 

 from the first. In this the former show a character that may be held as 

 primitive. 



Those superficial cells of the growing tips wdiich are thus the ultimate 

 parent cells of all the tissues are called the initial cells. They vary in number 

 and in form. In the Filicales it is found that their number is small, frequently 

 only a single cell. The form may be prismatic, or it may be conical with 

 two, three, or four sides. Differences in number and form such as these may 

 themselves be used for purposes of comparison and phyletic seriation, as will 



