716 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



By the elongation of this suspensor the embryo proper is brought down into the 

 aforementioned food-reserve, where it continues its development. This process is 

 quite similar to the corresponding stage in Flowering Plants, where the suspensor 

 is almost universally found. 



Lepidodendracece. — This family, represented only by fossils from the Devonian 

 and Carboniferous formations, consisted of large-growing Lycopod-like forms, with 

 huge stems clad with linear leaves. They exhibit a secondary growth in thickness 

 (wanting in recent Lycopods), and both micro- and macrospores were produced in 

 the cones. Casts of Lepidodendron-stems bear characteristic rhomboidal areas 

 corresponding to the leaf bases, and upon these the actual leaf -scars may be seen. 



Fig, 405.— Lycopodiales. 



' Isoetes lacustris. 2 Expaaded base of leaf showing the sporangium immersed io its socket and partly hidden by the velum 

 and the ligule above. 3 Longitudinal section of base of leaf showing the strands crossing the sporangium and the inser- 

 tion of the ligule. * Leaf from the cone of Lycopodium clavatum showing the kidney-shaped sporangium. ^ A single 

 spore of L. clavatum. * Prothallium of L. annotinum with young plant attached. 1 natural size ; 2, 3^ 4^ 6 x 10 ; ^ X 100. 

 (After Luerssen.) 



Sigillariacece. — Another family which flourished in carboniferous times. Like 

 the Lepidodendraceffl, their stems are gigantic, thickened and scarred; they were 

 also heterosporous. The marks on the stems are not rhomboidal, but shield-like, 

 and they stand in vertical rows. The curious branching remains named Stigmaria 

 constitute the root (or rhizome) of Sigillaria. 



IsoetacecB.^Ia a small family of aquatic mode of life, containing the single genus 

 Isoetes, which is represented by some 50 species in various portions of the globe. 

 Unlike the other members of the alliance Lycopodiales, Isoetes possesses an ab- 

 breviated stem, bearing a tuft of lance-like leaves. The common British species 

 Isoetes lacustris may serve as type of the genus (fig. 405 ^). It grows in quantity 

 in the mud at the bottom of upland tarns and lakes in the northern parts of the 

 country, and is attached by delicate roots which repeatedly fork. The very short 



