GROUP IV. GYMN'OSPERM^. 



433 



The conspicuous features of their morphology are the regular mono- 

 podial branching of the stem, the small (often acicular) simple leaves, and 

 the tap-root. In their histology, these plants resemble the Dicotyledons 

 in that the stem grows in thickness by a normal cambium-ring ; but the 

 vascular tissue of the wood consists entirely of tracheides with bordered 

 pits. The presence of resin-ducts is another characteristic feature. 



The flowers are never monoclinous; some genera are dioecious. The 

 microsporangiate flower is a cone, consisting of an elongated axis bearing 

 microsporophylls (Fig. 254), which are generally somewhat peltate in form. 

 Each microsporophyll bears two or more microsporangia on its under (dor- 

 sal) surface. The macrosporangiate flower is also a cone in certain cases 

 (Pinoideae, Fig. 255), in which case the macrosporophylls bear the macro- 



FIG. 254. Pinus montana (Pumilio). A Longitudinal section of a microsporangiate flower 

 ( x 10). B Longitudinal section of a microsporophyll, showing the cavity of one pollen-sac 

 (x 20). C Transverse section of a microsporophyll, showing the cavities of both pollen- 

 sacs. D Germinating two-celled microspore of Ptnus sylvestris, showing the expansions of 

 theexine(x 400). (After Strasburger.) 



sporangia; in other cases there is a less perfect cone, or none at all 

 (Taxoidese, see Fig. 258), the macrosporophylls are either rudimentary or 

 absent, and the macrosporangia are generally borne on the axis. 



In some genera (e.g. Pinus, Juniperus) the seed takes two years to ripen ; 

 in the first year, pollination takes place, and the pollen-tube begins to 

 grow through the tissue of the nucellus ; in the second year, after a period 

 of rest, the pollen-tube completes its growth, reaches the archegonium, and 

 fertilises the oosphere ; as a consequence, the embryo is developed, and the 

 ovule is changed into a seed. 



M.B. V F 



