XIV ISOETACE^ 551 



epidermis. From the inner one arises the initial cell (or cells?) 

 of the plerome, which soon becomes well defined and connected 

 with the primary strand of procambium in the axis of the em- 

 bryo. It is cjuite possible that here, as in the older roots, a 

 single initial cell is present in the plerome, but this is not cer- 

 tain. The layer of cells immediately below the primary epi- 

 dermis is the initial meristem for all the tissues of the root 

 except the plerome. The primary epidermis later divides into 

 two concentric layers which take no further part in the growth 

 of the root except as they join the outer layers of the root-cap. 



From the layer above the plerome initial, additions are made 

 at regular intervals to the root-cap, and these layers remain one 

 cell thick, so that the stratification is very marked. At the 

 apex of the root there is no separation of dermatogen and peri- 

 blem, which are first differentiated back of the apex. The pri- 

 mary xylem consists of very delicate spiral tracheids formed at 

 the base of the root at the same time that the first ones appear 

 in the leaf. 



The foot increases much in size as the leaf and root develop, 

 and its superficial cells become much enlarged and encroach 

 upon the large cells of the prothallium, whose contents are 

 gradually absorbed by it. 



The cotyledon is at first composed of compact tissue, which 

 during its rapid elongation separates in places, and forms a sys- 

 tem of large intercellular spaces. There are two rows of very 

 large ones, forming two broad air-chambers extending the 

 whole length of the leaf, but these are interrupted at intervals 

 by imperfect partitions composed of single layers of cells. In 

 the root there are similar lacunae, but they are smaller and less 

 regularly arranged. 



The growing embryo is for a long time covered by the pro- 

 thallial tissue, wdiich in the upper part continues to grow with 

 it; but finally cotyledon and root break through, the former 

 growing upward, the root bending down and anchoring the 

 young sporophyte in the mud. Owing to the large air-spaces 

 the cotyledon is lighter than the water, and always stands ver- 

 tically, whether the original position was vertical or horizontal. 

 In the latter case the plant appears to be attached laterally to the 

 prothallium, and the stem apex, which when first formed stands 

 almost vertically, now assumes the horizontal position which 

 it has in the older sporophyte. 



