386 



A TEXTBOOK OF THEORETICAL BOTANY 



with the nucleus of the oosphere. In one species parthenogenesis occurs 

 in nature, while it has been artificially induced in others. 



After fertilization the zygote nucleus passes to the apex of the oosphere, 

 which secretes a cellulose wall and becomes an oospore with a yellow or 

 brownish coloration. It is still enclosed in the enveloping cells which now 

 contribute to the wall of the oospore. Their inner walls become suberized, 

 while the outer walls form the sculpturing on the surface of the oospore. 

 Lime is also deposited, so that the mature oospore is a very resistant structure. 

 At this stage the oospore is shed, and contains both starch and oil as food 

 reserves. 



Development of the Protonema 



The reserve of food material in the oospore recedes to its basal end, while 

 the nucleus passes to the apical end and divides by two successive divisions, 

 which are thought to be reductional, to give four nuclei. A septum now 

 appears and cuts off one of these nuclei in an apical cell, while the other 

 three disintegrate. The enveloping wall bursts into five or six segments to 

 expose this upper cell, which then divides longitudinally into two cells, 

 which elongate and grow in opposite directions to form the primary rhizoid 

 and protonema respectively (Fig. 370). This protonema becomes green 

 and undergoes repeated transverse division into a filament of cells. Of these, 

 the lowest undergoes no division and remains colourless. It is regarded as 

 the basal cell, but it elongates, thus carrying the remaining cells up with 

 it. Above this are at first three cells, which are the root-node cell, the 

 internodal cell and the stem-node cell respectively. 



OOSFORE 



PROTONEMA n^ I 



-^1 



FIRST NODE-^ 



internode 



oospore 



root 



NODE 

 CELL 



ACCESSORY 

 RHIZOID 



BASAL CELL 



PRIMARY 

 RHIZOID 



I SECONDARY 

 » RHIZOIDS 



Fig. 370. — Chara vulgaris. A and B, Early stages in the germination of the oospore. 

 Chara canescem. C and D, Growth of protonema and primary' rhizoid and the 

 differentiation of the embryo. Tolypella glomerata. E, Septation of embryo 

 and formation of accessory rhizoid. Chara fragilis. F, Fully developed 

 embr\-o. {From Grove and Bullock Webster.) 



