144 THE PLANT CELL. 



iiection with the formation of the embryo-sporophyte, it should 

 be mentioned that the young tissues are produced by the 

 divisions arising in what is known as an apical cell. This cell 

 is pyramidal in shape, with the base outwards, and walls are 

 produced in it, parallel to the three sides of the pyramid. Fresh 

 tissues, even lateral buds, all have this form of cell at their 

 apices, and growth is thus entirely apical at first, the subsequent 

 walls arising in other jjlanes. The apical cell is found not only 

 in the Pteridophyta, but also in the Bryophyta, and is typical 

 of both these groups of plants. 



Note. — The study of the reproductive cycle in the Homosporous 

 Pteridophyta may be carried out in Pleris or Aspidium, (FilicineEe). The 

 spores are readily examined by brushing off a number of sporangia into a 

 drop of water on a slide, and observing rapidly under the microscope, 

 when the annuli «'ill stretch open, and the spore-chambers rupture, 

 freeing the spores. 



The prothalli are best examined by growing spores on moist humus, 

 so prepared as to exclude moulds, and watching the stages of growth, in 

 order to pick out prothalli showing the various phases in the formation 

 of the antheridia and oogonia, and, later on, of the embryo-sporophyte. 

 Fresh gametophytes may be examined in glycerine and water, or the 

 structures may be fixed and hardened and sections taken in split pith, or, 

 after embedding in celloidin, by means of a special microtome. 



II. Reproduction in the Heterosporous Pteridophyta (types 

 Marsilea, Salvinia). — A brief description of this type of repro- 

 duction in Pteridophyta is necessary on account of the important 

 comparisons to be made between it and that occurring in Angio- 

 sperms and Gymnosperms. 



In Marsilea, one of the Hydropteridese, two kinds of sporangia 

 are found — viz., microsporangia and macrosporangia, in special 

 organs, the sporocarps. In the former, a number of small spores, 

 or microspores, are produced; and in the latter, a few large 

 spores, or macrospores, arise. 



A microspore, on being freed by the rupture of its sporan- 

 gium, germinates, and produces a small male prothallium, which 

 is enclosed within the limits of the thick outer coat or exospore 

 of the microspore. Upon this prothallium, antheridial cells are 

 formed, in which antherozooids arise, somewhat after the same 

 manner as those of Aspidium. A macrospore, when freed from 

 its sporangium, also germinates, and gives rise to a somewhat 

 larger female prothallium, upon, or in, which an oogonium is 

 formed, containing, after the cutting off of certain canal- 

 cells, the oosphere. There are thus two separate gametophytes 



