498 



BOTANY 



PART II 



for fertilisation. The spermatozoids are induced to direct their motion 

 toward the archegonia by the excretion from the latter of a substance 

 which diffuses into the surrounding water. In Ferns, Salvinia, 

 Equisetum, Selaginella, and Isoetes, this substance is malic acid or one 

 of its salts, while in Lycopodium it is citric acid. 



Other organic acids, some salts of the metals, and even some alkaloids may 

 serve as attractive substances. Differences exist in the behaviour of different 

 genera in this respect. The chemotactic sensibility of the spermatozoids may 

 exist for a number of substances ( n3 ). 



After the fertilisation of the egg-cell by a spermatozoid there is 

 developed from it, as in the Bryophyta, the diploid asexual genera- 

 tion ; this is the cormophy tic 

 fern-plant. 



The asexual generation 

 or sporophyte is represented 

 by a plant possessing a highly 

 differentiated internal struc- 

 ture, and externally seg- 

 mented into stem, leaves, 

 and roots. In the majority 

 of Pteridophytes (Ferns, 

 Equisetum), the fertilised 

 egg-cell, while still in the 

 archegonium, surrounds it- 

 self with a cell wall and 



FIG. 463. Transverse section of the rhizome of Pteri- , , . . . n , . , 



dium aquilinum. f, Concentric vascular bundles; undergoes dlVlSlOIl, fiPgt IDtO 

 , sclerenchymatous plates; sp, peripheral zone of tWO Cells, by the formation 



of a basal wall, and then 

 into octants by two walls 

 at right angles to each other and to the basal wall. By the 

 further division of these eight cells a small mass of tissue is formed, 

 and from this are developed the stem apex, the first leaf, the 

 primary root, and an organ peculiar to the Pteridophytes, the so- 

 called FOOT (Fig. 462/). The foot is a mass of tissue/- by means 

 of which the young embryo remains attached to the parent prothallium 

 and absorbs nourishment from it, until, by the development of its 

 own roots and leaves, it is able to nourish itself independently. In 

 some Lycopodineae (Lycopodium, Selaginella) a suspensor consisting 

 of one or a few cells is formed and serves as an absorbent organ. 

 The prothallium usually dies after the development of the young 

 plant. The stem developed from the embryonic rudiment may be 

 either simple or bifurcated, erect or prostrate ; it branches without 

 reference to the leaves, which are arranged spirally or in whorls, 

 or occupy a dorsiventral position. Instead of rhizoids, as in the 

 Bryophyta, true roots are produced, as in the Phanerogams. The 



sclerenchymatous fibres ; r, cortex ; e, epidermis, 

 (x 7.) 



