THE YOUNG SPOROPHYTE 



71 



in the very young leaf, can no longer be recognized and the lamina is divided into 

 two nearly equal lobes, of which one is lateral with reference to the other, but these 

 two lobes probably arise as a result of the dichotomy of the original apex. A group 

 of narrow marginal cells somewhat to one side of the terminal lobe evidently consti- 

 tuted a meristem tissue, and this was preparing for the second dichotomy, by which 

 the ternate form of the young leaf is established. The branching of the lamina at 

 this early period therefore appears to be the result of an unequal dichotomy, as it is 

 in the first foliage leaf. The vascular bundles from each of the two primary leaf seg- 

 ments were just beginning to be recognizable and join into one at the base of the 

 lamina. Surrounding the leaf is the sheath developed from the base of the second 

 leaf. This sheath in the section appears as a closed ring some two or three cells in 

 thickness and looking very much like the sheath of Ophioglossum. The top of the 

 conical sheath is closed and does not show the pore found in Ophioglossum. The 

 sheath becomes thicker lower down, where four rows of cells show in the section. 

 Between the inclosed leaf and the surrounding sheath in the lower portion (fig. 49, B) 

 may be seen the section of flattened scales (sc) which grow from the base of the leaf 



Fig. 4 V 



A, B. Two sections of terminal bud of a young sporophyte of Helminthosla 



C, D. Two similar sections from an older plant, .v, stem apex; sh, petiolar sheath. 



and from the tissue immediately about the stem apex. At its base the sheath of 

 the third leaf is already completely developed and surrounds the young fourth leaf 

 (/ 4 ), which in this section was cut horizontally and shows plainly its large apical cell. 

 The stem apex, which lies directly below the younger leaf, was cut obliquely in this 

 series and did not show the form of the apical cell plainly, but in a young plant of 

 about the same age (fig. 47, C) the apical cell showed that it was regularly trian- 

 gular in form, seen in cross-section. 



At the level of the stem apex the second leaf shows a single vascular bundle 

 which may be continued undivided upward, or it may divide into two bundles in the 

 petiole. In regard to the structure of the petiole, therefore, Helminthostachys is to 

 some degree intermediate between Botrychium and Ophioglossum. I he young trace 

 of the third leaf can be seen at this level as an oval mass of undifferentiated cam- 

 bium. Immediately below the stem apex the cells are smaller than is the case in 

 the young sporophyte of Botrychium, but one can nor make out the beginning of a 

 central procambium cylinder. The smaller cells in the region immediately underlying 

 the stem apex may be explained by the fact that in these young plants there is no 

 pith developed within the axial stele as there is later on. I he youngest root in this 



