248 BOTANY 



Osraunda the first division of the spore may take place within less 

 than twenty-four hours. Where chlorophyll is not present the pro- 

 cess of germination is retarded, as chlorophyll must be developed 

 before any further growth takes place. 



Germination. In most Ferns the first division in the germinating 

 spore (Fig. 212) cuts off a small cell, which at once lengthens and 

 forms the first root-hair, from a larger green cell which gives rise to 

 a row of cells varying in length under different conditions. In the 

 terminal cell of the row a two-sided apical cell is formed by inter- 

 secting oblique walls, and the gametophyte rapidly forms a fiat 

 thallus. New root-hairs grow out from the lower side, and fasten 

 it to the earth. At this stage the young gametophyte closely resem- 

 bles a simple thallose Liverwort, such as Metzgeria or Aneura, and 

 as in these, the apical cell lies in an indentation of the margin of the 

 heart-shaped thallus, caused by the rapid growth in the outer cells of 

 the young segments cut off from the apical cell. 



Sooner or later the two-sided apical cell is divided by a transverse 

 wall, and from this time inner or basal segments are regularly cut 

 off, which undergo horizontal divisions, so that back of the apex the 

 thallus forms a cushion of tissue, upon whose lower surface the arche- 

 gonia are later developed. If this thickening begins early, as in 

 Osmunda and Marattia, a thickened midrib is developed. The root- 

 hairs develop little or no chlorophyll, but the other cells contain 

 numerous discoid chromatophores. 



Both archegonia and antheridia are borne upon the same plant in 

 most Ferns, but some are dioecious (e.g. Struthiopteris Germanicd). 

 In the latter the male plants are smaller and less regular in form 

 than the females. Small male plants are not uncommon in species 

 which also produce large hermaphrodite prothallia. 



The largest gametophytes are found in certain tropical Ferns, 

 especially species of Vittaria and Hymenophyllum, in which the 

 gametophyte may reach a length of several centimetres and fork 

 repeatedly, like a thallose Liverwort, which it closely resembles. 

 These prothallia often increase in numbers rapidly by the develop- 

 ment of gemmae, and thus they may occur in great numbers, forming 

 thick mats upon the trunks of trees, or upon rocks. 



The Sexual Organs 



Antheridium. The antheridium in the lower Ferns shows a slight 

 analogy with that of the Anthocerotales, especially those forms in 

 which a single antheridium is developed in each cavity. As in the 

 Anthocerotales, the cell from which the antheridium develops arises 

 by a transverse division of a superficial cell into an outer and an 

 inner cell. The latter, which in the Anthocerotales gives rise to the 



