328 BOTANY 



and bast. In addition to the primary ring of bundles, there may also be devel- 

 oped accessory bundles, both in the pith and cortex, resulting in a very compli- 

 cated arrangement of the tissues in the older stem. The bundles are always 

 collateral, and in structure approach most nearly those of the eusporangiate 

 Ferns. Between the primary vascular bundles are extensions of the pith, or 

 medullary rays, such as are found in the stem of the Coniferae, but which are 

 also found in Botrychium. There is developed in the outer cortex a mass of 

 growing tissue, or Periderm. 



The Leaf 



Two vascular bundles enter each leaf, and fork several times, so that a cross- 

 section of the petiole (Fig. 289, B) shows several bundles arranged in the form 

 of an O. Branches are given off into each leaflet, where they may remain un- 

 divided (Cycas), or may divide. 



Mucilage-ducts. Traversing the petiole are numerous conspicuous gum and 

 mucilage-ducts (m) which resemble very closely those of the Marattiacese. 



Vascular Bundle. The bundles of the petiole in Cycas (Fig. 289, C) show a 

 group of small spiral tracheids near the centre, outside of which is amass of large 

 scalariform tracheids. The rest of the bundle is composed of the phloem, which 

 contains large sieve-tubes with lateral sieve-plates, somewhat like those of the 

 Ferns. There is a conspicuous bundle-sheath (en), associated with which are 

 often encountered cells containing crystals. In Dioon, concentric bundles, also, 

 are found in the petiole. The green tissue of the leaf forms a palisade-paren- 

 chyma above, the cells having their walls more or less thickened so as to give 

 firmness to the leaf. The stomata, which show accessory cells about the guard- 

 cells, are confined to the lower surface. 



The Root 



The primary root of the embryo (Fig. 292, E) develops into a thick tap-root, 

 as it does in the Conifers, and like these, it shows a secondary thickening due to 

 the development of a cambium. 



Tubercular Roots. Curious secondary roots are sometimes developed, espe- 

 cially in Cycas. These arise near the surface of the ground and show a dichoto- 

 mous branching, which results in dense coral -like masses. Associated with these 

 are always found Bacteria and a species of Anabsena. These root-tubercles 

 always contain large air-spaces, and may perhaps be special organs for aeration 

 of the roots. The presence of the Schizophy tes within the tissues may possibly be 

 associated with the assimilation of nitrogen. 



The Sporangia 



The macrosporangia (ovules) and microsporangia (pollen-sacs) are in all 

 Cycads borne upon different plants. In Cycas the macrosporophylls (PI. IV) are 

 separate, and arranged like the foliage-leaves, which they also resemble in their 

 pinnate form. . In all of the other genera both kinds of sporophylls are arranged 

 in a thick cone (Fig. 290, F) upon special shoots. 



The microsporangia are borne upon the lower side of the sporophyll (Fig. 

 290, C), and may be arranged in sori like those of a Fern. In their origin and 

 development they are strikingly like the sporangia of the Marattiacese, even 



