3H FOURTH GROUP. SEED-PLANTS. 



inwards, while the pinnae are straight. The leaf is pinnate, in the genus Bowenia 

 bi-pinnate. The usually sessile pinnae are marked as regards their venation by the 

 absence of anastomoses, the frequence of dichotomy and the uniformity in size of the 

 veins, except in Bowenia ; these characters are of importance in determining fossil 



remains l . 



The stem when young is like a tuber in form, and in some species it retains this 

 character to a later time. It seldom attains any great height (Cycas\ and is usually 

 unbranched, as are the similarly growing stems of Aspidium Filix-mas, the Ophio- 

 glosseae, Isoetes, and some others, where also the apex of the stem which elongates 

 very slowly has a considerable breadth. The stem of the Cycadeae has still greater 

 likeness to that of the Tree-ferns, which without forming internodes is thickly beset 

 with leaf- scars and branches from the leaf-stalks; and like it the cycas-stem also 

 enlarges considerably close beneath the apex at an early stage of its growth, and 

 subsequently increases very little in thickness. The anatomical structure will be 

 briefly noticed further on. 



On the other hand the Cycadeae are distinguished from all Vascular Cryptogams 

 by the presence of a tap-root. Secondary roots appear above the ground and branch 

 dichotomously. Anabaena, one of the Nostocaceae, is often found in the intercellular 

 spaces of the roots, where its presence causes tubular protuberances in the adjacent 

 cells 2 , but is not the cause of the forked branching, such as is produced for instance 

 in the roots of the Coniferae by the mycelia of Fungi. 



The disposition of the flowers of the Cycadeae is always dioecious, the plants 

 themselves are therefore male or female ; both kinds of flower appear on the summit 

 of the stem, either singly, as in Cycas, as the terminal flower of the stem, or in pairs 

 or more together, as in Zamia muricata and Macrozamia spiralis, perhaps as meta- 

 morphosed bifurcations of the stem. The flower consists of a stout conical elongated 

 axis which in its lower part is sometimes a naked stalk, but elsewhere is furnished 

 with numerous crowded leaves arranged spirally and bearing macrosporangia or 

 microsporangia. The flowers of Cycas are therefore not essentially distinct from 

 the sporangiferous spikes of many Vascular Cryptogams. 



The female flower in Cycas is a rosette of foliage-leaves of the stem slightly 

 metamorphosed ; the apex of the stem produces above them first scale-leaves, then 

 fresh foliage-leaves, then scale-leaves and leaves bearing sporangia, as sterile and fertile 

 leaves alternate in some Ferns, Struthiopteris for example. The stem therefore grows 

 through the female flower 3 . It is true that the special leaves that bear the macrospo- 

 rangia are much smaller than ordinary foliage-leaves, but their form and structure are 

 essentially the same. The lower pinnae are replaced by macrosporangia (ovules) 



1 [Bower, Comp. Morph. of the leaf in Vase. Crypt, and Gyrnnosp. (Phil. Trans. 1884), treating 

 the whole leaf as a branch-system shows that the rounded apex of the phyllopodium (i. e. the main 

 axis of the leaf exclusive of pinnae) is covered with a definite dermatogen-layer ; its growth is never 

 very distinctly apical, more so in Cycas and perhaps in Dioon than in other genera ; it is winged 

 throughout ; its branching is monopodial, the order of succession with few exceptions being basipetal. 

 This view of the leaf as a phyllopodium is quite at variance with that advanced in the text, which 

 assumes a lower portion of a leaf, a leaf- base, which in the scale-leaves is alone developed, distinct 

 from an upper portion.] 



2 Reinke in Bot. Ztg. 1879, P- 4/3- 



3 It is true that this expression 'growing through 1 does not mean the same thing exactly as in 

 Angiosperms. 



