530 



BOTANY 



PAKT II 



The development of the embryo, in which a suspensor consisting of one or 

 several cells, the apex of the stem with the first leaves, the first rhizophore and 

 the foot are distinguishable, proceeds in a great variety of ways in the genus. 



The first division in the fertil- 

 ised egg-cell is transverse. In 

 S. Martensii, spinulosa, hel- 

 vetica, etc. , the upper hypobasal 

 cell gives rise to the suspensor 

 only, the main portion of the 

 embryo being derived from the 

 lower cells (Fig. 498) ; in S. 

 denticulata the upper cell forms 

 the foot and rhizophore as well 

 as the suspensor. The apex 

 of the shoot with the first pair 

 of leaves grows upwards and 

 the root downwards ; the young 

 plant remains attached to the 

 prothallus in the megaspore by 

 the foot (Fig. 494 ). In 



FIG. 498. Selaginella Martensii. Embryo before becoming S0 me species (S. rubricaulis, 

 free from the prothallus in longitudinal section ; /, foot; spinulosa) the arch e g onia re- 

 main closed and the egg de- 

 velops apogamously into the 

 embryo. In S. Kraussiana and related forms the suspensor, according to Bruch- 

 mann, is reduced, but replaced functionally by a special embryonic tube proceeding 

 from the wall of the mother cell of the ovum ; the embryo is delimited in this 

 and comes into relation with the nutritive tissue. 



wt, rhizophore; et, suspensor; fc, cotyledons with their 

 ligules. (x 150. After BRUCHM ANN.) 



Order 4. Isoetaceae ( 1S1 ) 



The isolated genus Isoetes must be regarded as a persistent branch of an 

 ancient group of plants, which in earlier geological periods was more richly 

 represented. The species of Isoetes are perennial plants, growing either on 

 damp soil or submerged in water. The stem is short and tuberous, rarely 

 dichotomously branched, terminating below in a tuft of dichotomously-branch- 

 ing roots, and above in a thick rosette of long, stiff, awl-shaped leaves 

 (Fig. 499). The stem is characterised by a secondary growth in thickness by 

 means of a cambium ; this produces to the outer side cortex (without phloem) 

 and to the inner side secondary phloem and xylem. The leaves are traversed 

 longitudinally by four air-passages, and expand at the base into a broad sheath. 

 On the inner side of the leaves, above their point of insertion, is an elongated 

 pit, the fovea, containing a large sessile sporangium. A ligule, in the form of 

 a triangular membrane, is inserted above the fovea. Isoetes thus differs greatly 

 in habit from the other genera, but resembles Selaginella in the development of a 

 ligule. On this account Isoetes and Selaginella are termed Ligulatae ; the extinct 

 Sigillariaceae and Lepidodendraceae also belong to this group. 



The macrosporangia are situated on the outer leaves of the rosette ; the micro- 

 sporangia on the inner. Both are traversed by transverse plates of tissue or 

 trabeculae, and are in this way imperfectly divided into a series of chambers. In 

 contrast to Selaginella numerous macrospores are formed in each macrosporangium. 

 The spores are set free by the decay of the sporangial walls. 



