Januaey 16, 1903.] 



SCIENCE: 



95 



In short, while we know to a consid- 

 erable extent some of the factors which 

 determine the formation of sexual ceUs, 

 where these have already been developed, 

 the reasons why sex has developed are 

 still very obscure. 



Secondary reproductive structures, such 

 as sporangia, seeds, flowers, fruit, etc., are 

 readily enough explicable and need not be 

 dwelt upon here. 



PHOTOSYNTHESIS. 



Perhaps the most important physiolog- 

 ical property of green plants is the photo- 

 synthesis, or the ability to utilize the en- 

 ergy of the sun's rays for the manufacture 

 of the primary carbon compounds neces- 

 sary to build up living protoplasm. That 

 some of the most striking modifications of 

 the plant body are directly associated with 

 photosjrnthesis is certain. The develop- 

 ment of leaves in various groups of plants 

 is, perhaps, the most obvious response to 

 the needs for photosynthesis. The leaf is, 

 par excellence, the photosynthetie organ. 

 The spreading out of the green cells so as 

 to offer the most favorable exposure to 

 the light rays, and in the higher plants 

 the development of stomata and the spongy 

 mesophyll, or special assimilating tissues, 

 are especially perfect. Leaves are by no 

 means confined to the vascular plants, 

 however. We need only recall the simple 

 leaves of mosses and liverworts and the 

 similar organs in the more highly organ- 

 ized seaweeds, such as Sargassum or Macro- 

 cystis. Even among the truly green algse 

 simple photosynthetie organs may be de- 

 veloped. The dense branching tufts of 

 Draparnaldia or the expanded frond of 

 Ulva, for example, are of this nature. 



The leaves of these lower plants are very 

 different morphologically from those of 

 the ferns and seed plants, but show very 

 clearly that they are physiologically of the 



same nature; i. e., they are analogous but 

 not homologous. 



Other special modifications associated 

 with photosynthesis are the peculiar la- 

 cunar tissues found ia the thallus of the 

 Marchantiales and iu the sporogonium of 

 the true mosses and in Anthoceros. In all 

 these instances there are formed, ia con- 

 nection with the green lacimar tissue, more 

 or less perfect stomata. These upon the 

 apophysis of the sporogonium of many 

 mosses, and over the whole surface in 

 Anthoceros, are precisely similar to those 

 found upon the leaves and other green 

 organs of the vascular plants. 



While it is usually stated that, among the 

 bryophytes, appendicular organs are quite 

 absent from the sporophyte, the apoph- 

 ysis, or special assimilative organ at the 

 base of the capsule in some of the more 

 specialized mosses like Polytrichum and 

 Splachnum, might almost be so regarded. 

 In the latter genus it sometimes forms a 

 broad disk several times the diameter of 

 the rest of the capsule, and is just as truly 

 a special organ for photosynthesis as is the 

 leaf of a fern or flowering plant. 



WATER. 



Even more important than the changes 

 of the plant body associated with photo- 

 synthesis are those which are due to the 

 plant's relation to the water supply. All 

 organisms require a certaiu amount of 

 water in order that the protoplasm may 

 perform its functions. Protoplasm is not 

 necessarily killed by the withdrawal of 

 water, but it is rendered inactive, as may 

 be readily seen in such structures as seeds, 

 spores, etc. 



The lowest organisms, whether plant or 

 animal, are virtually aquatic ; for, although 

 they do not necessarily always remain in 

 a liquid medium, they become quiescent 

 when moisture is withheld. Very many, 

 like most algse, are true aquatics, and it 



