SELAGINELLACE^ 41 



of a small number of large cells, known as the siispensor, or pro-embryo, 

 from its apparent homology with the structure which goes by this name 

 in Flowering Plants. By the elongation of the suspensor and the 

 compression and absorption of the adjacent cells, the lower portion of 

 the oosperm is forced into the endosperm, from which it appears to 

 derive nutriment, dividing, both previously and subsequently, into a 

 small-celled tissue. This tissue undergoes a large amount of differentia- 

 tion before the embr}-o emerges from the megaspore, the rudiments of 

 all the principal parts of the sporophyte making their appearance at this 

 early stage. The mother-cell of the embryo divides into two by an 

 oblique wall. From one of the two cells thus formed originate the stem 

 and one of the coty/edons, from the other the foot and the other cotv- 

 ledon. The rudimentary stem has a two-edged apical cell, from which 

 segments are cut off alternately right and left. An inner mass of tissue 

 soon becomes differentiated as the procambium of the axial ' vascular ' 

 bundle, the peripheral tissue as dermatogen and periblem. The stem- 

 bud, or plumule, with its first leaves (subsequent to the cotyledons), 

 finally grows erect from the apex of the megaspore as the embr}'o 

 increases in length. The formation of the first root begins later between 

 the foot and the suspensor ; its apical cell is formed from an inner cell 

 of the older segment ; the first layer of its root-cap originates from the 

 splitting into two layers of the overlying dermatogen ; the later layers of 

 the root-cap are formed from the apical cell of the root. 



The sporophyte differs greatly in appearance in the two orders. 

 In Selaginella the stem is always slender, erect or procumbent, with 

 distinct internodes, and lengthening rapidly by monopodial branching, 

 which very often has a dichotomous appearance from the vigorous 

 growth of the lateral branches. These lateral branches, with their 

 ramifications, frequently develop in a single plane, giving the system 

 the appearance of a compoundly pinnate leaf; all the branches and 

 leaves displaying a distinctly dorsiventral character. The stem has an 

 epiderm composed of elongated prosenchymatous cells, without 

 stomates, but containing a considerable amount of chlorophyll in re- 

 markably large grains. The fundamental tissue consists of elongated* 

 thin-walled cells with oblique septa, fitting closely together without 

 intercellular spaces, and endowed with the power of long-continu^ 

 growth both in length and diameter. In the absence of small inter- 

 cellular spaces the stem of Selaginella resembles that of mosses ; but, 

 on the other hand, each 'vascular' bundle is surrounded by a large 

 air-cavity, traversed by trabecules, rows of cells connecting the bundle 

 with the surrounding fundamental tissue. The entire cortex has a 

 tendency to become dark brown with age from partial'' sclerosis. AVhen 



