PHANEROGAMIA. 717 



stem is furrowed on two sides, and from these furrows arise the roots. On rare 

 occasions it is branched. From this stem arise numerous pointed leaves, which 

 are slightly expanded below. Each leaf bears a sporangium, immersed in a socket 

 on its upper surface (fig. 405 ^), and partly covering the sporangium a membrane, 

 the velum. Immediately above the sporangium is the ligule, a little tongue-like 

 emergence of unknown function (figs. 405 ^ and 405 ^). Macrosporangia, containing 

 several large macrospores, are generally found on the outer leaves, and micro- 

 sporangia, with very numerous microspores, on the inner onos. The sporangia are 

 crossed by strands of cells (c/. fig. 405 '^), termed trabeculee, but these do not 

 partition them into chambers. The germination of the spores presents certain 

 resemblances to the same event in Selaginella, but it cannot be followed out in 

 detail here. 



An interesting feature in the structure of Isoetes is the existence of a cambium- 

 Uke zone in the stem just outside the central bundle-cylinder. This adds new tissue, 

 both towards the inside and outside, but most abundantly towards the outside. 

 This latter secondary cortex is parenchymatous, but in time it becomes corky. 

 To its formation is due the curious form of the stem. 



Though many species of Isoetes live below water, others are terrestrial or 

 semi-aquatic in habit. /. lacustris, as it grows at the bottom of a mountain tarn, 

 is very similar in general appearance to two flowering-plants which affect the same 

 situation, viz.. Lobelia Dortmanni and Littorella lacustris; a closer inspection, 

 however, will readily distinguish it. 



Phylum IV.— PHANEROGAMIA, Flowering Plants 



The general characters of Flowering Plants have been so fully dealt with in 

 previous sections of this work that little more is needful here beyond a bare outline 

 of the classification of their divisions and alliances. 



The Phanerogamia are characterized by the production of seeds. The macro- 

 sporangia of heterosporous Archegoniatee are here represented by ovules, the macro- 

 spores by embryo-sacs, and the microspores by pollen-grains. The macrospore 

 (embryo-sac) remains inside its sporangium (ovule), and here produces the reduced 

 female prothallium (endosperm), which never has an independent existence. An 

 egg-cell is formed within the embryo-sac, and this is fertilized by the pollen-tube 

 which has arisen from a pollen-grain lodged upon a suitable receptive surface in 

 the vicinity of the ovule. Ultimately, after the embryo has attained a certain 

 differentiation, the whole macrosporangium, with contained embryo and food- 

 material, comes away, and is known as the seed. 



The oophyte or prothallial generation is thus suppressed as an independent stage 

 in the life-cycle. The sporophyte, on the other hand, attains to a markedly more 

 complex development than in the groups already treated. Fertilization of archegonia 

 on free-growing prothallia by swimming spermatozoids is here replaced by a direct 

 penetration of pollen-tubes to the ovules. To the " flower " also new duties arc 



