uo 



THE PLANT CELL. 



I. Reproduction in the Homosporous Pteridophyta {Aspidium). 

 — The stages and structures to be examined here are : — 



a. The spore and seXual generation (gametophyte or pro- 

 thallus). 



b. The sexual organs arising on the aexual generation, and the 

 essential cells formed in these organs. 



c. Fertilisation and the origin of the sporophyte or asexual 

 generation. 



These will be considered in detail. 



a. The Spore and Sexual Generation (Gametophyte). — The 

 spores are formed in certain well-defined structures known as 

 the sporangia. These arise either from one epidermal cell 

 or a f/roiqj of cells upon small cushions of tissue, tire sori, 

 which are formed upon the under surfaces of the sporophylls 

 or spore-bearing fronds of the fern. Usually these sori are 

 found at the endings of the lateral branches of the leaf-traces of 

 the frond, and their position varies according to the genus. 

 A sheath of cells known as the indusium often covers over each 

 sorus, but in some cases the sori are naked. Each sporangium 

 is composed of three parts (see Fig. 107), viz.: — 



i. The stalk, which grows from the sorus." 



ii. The spore-ehamber or sporangium proper, whieh is thin- 

 walled, and composed of small translucent cells. Inside the chamber are 

 seen the sporeS, which are produced at an early stage by the divisions of 

 an archesporial cell which forms the spore mother-eells, each of these 

 latter going to produce foUP spores. 



iii. The annulUS, a curved portion at the back of the spoi'angium. 

 This is composed of peculiar cells, each having thick walls perpendicular 

 to the surface, and thinner very elastic outer walls. The annulus acts as 

 an elastic layer which helps to stretch open the spore-chamber when the 

 thin front wall ruptures t and sets free the spores. 



A single mature spore is a simple spheroidal cell which 

 possesses two walls, an outer thick wall and an inner thin one. 

 Internally are cytoplasm, large nucleus, and a few food-granules 

 (starch). (See Fig. 107, 2.) At a certain period, determined 

 by the relative humidity of the atmosphere, rupture of the thin 

 anterior wall of the s2)orangium occui's, and the spores are freed, 



* A stalked gland arises in some cases from the slalk close to the 

 Kporangium proper, and is characteristic of the species .liipidium Jilix-nia^. 

 It is not represented in Fig. 107. 



t In connection with the rupturing of the anterior wall two peculiarly- 

 shaped cells are found, between which the rupture occurs along the middle 

 lamella. The two cells constitute the so-called " stomiuin. " 



