DIVISION IV. 

 GYMNOSPERM^:. 



The following- characters should be added to those already given 

 on page 2 : 



The Gymnos perms comprise only trees or shrubs. The flowers 

 are always unisexual and destitute of perianth (except Gnetacect) ; 

 the female plant of Cycas is the only one which has no flower. 

 The JIALK FLOWERS are constructed on the same type as the cones of 

 the Horsetails and Club-Mosses, and are most frequently lon</ shoot* 

 (Figs. 243, 258, "260 A, 267 J) bearing a number of spiral or verti- 

 cillate stamens. The FEMALE FLOWERS are of a more varied struc- 

 ture (see the orders). The OVULE is orthotropous (except t'odo- 

 carpus which is anatropous) and projects from the carpel up- 

 rightly, inverted, or horizontally; it has usually only one integument 

 (compare however Taxacea?) which proceeds from the upper part 

 of the nucellus, so that the embryo-sac in part is placed below 

 the integuments (Figs. 251, 264). The drop of mucilage which 

 catches the pollen-grains dries up and draws the pollen-grain 

 through the micropyle to a space just above the nucellus the 

 pollen-chamber in which the germination of the pollen-grain 

 commences. 



In each seed, only one of the many embryos which are formed 

 proceeds to its full development. The seed is always endospermous, 

 and the embryo has one, two, or a whorl of several cotyledons. 

 A vigorous primary root is developed on germination. THE 

 VASCULAR BUNDLES in the stem are arranged in a ring, and increase 

 in thickness takes place by a closed cambium-ring which forms 

 bast (phloem} externally, and wood (xyhm) internally with distinct 

 annual rings, as in the Dicotyledons. Only certain of the Cycadea 1 

 deviate from this arrangement. The secondary wood is very uni- 

 form, as it is almost exclusively formed of tracheides with bordered 

 pits, but true vessels are wanting \ this also indicates a relationship 

 with the Pteridophyta (see page 202). 



251 



