484 BOTANY PART n 



the upper tier being continuous with the remaining portion of the ovum. In the 

 further development of the three lower tiers the middle tier elongates to form the 

 SUSPENSOR, pushing the terminal tier from which the embryo will arise into the 

 tissue of the prothallium or endosperm ; the cells of the latter are filled with 

 nutritive reserve material. 



In other genera a separation of the four rows of cells takes place, and each bears 

 a young embryo. As a rule, however, only a single embryo continues its develop- 

 ment in each macrospore although several archegonia may have been fertilised. 

 The embryo consists of a main root or RADICLE directed towards the micropylc. 

 an axis, the HYPOCOTYL, a whorl of from two to several COTYLEDONS and the 

 i-i.rMULE or bud of the future shoot. 



The asexual generation which shows less uniformity will be described under the 

 separate orders. 



Order 1. Cycadinae ( :> ) 



This includes the single Family, Cyeadaeeae. These are woody 

 plants restricted to tropical and sub-tropical regions. The stem, 

 which undergoes secondary growth in thickness, is as a rule un- 

 branched and bears large, pinnate foliage leaves. These, which are of 

 firm leathery texture and persist for a number of years, alternate with 

 smaller scale-leaves and form a large terminal crown. The surface of 

 the cylindrical or tuberous stem is clothed with the scale-leaves and 

 the bases of the old foliage leaves. Mucilage ducts are present in 

 all parts of the plant. The vascular bundles are collateral but their 

 xylem consists of tracheides only. 



The Cyeadaeeae are dioecious. Fig. 446 represents a female plant 

 of Cycas revoluta, in which the growing point forms alternate zones of 

 foliage leaves and macrosporophylls. When young the foliage leaves 

 are rolled up circinately as in the Ferns. One of the sporophylls is 

 represented in detail in Fig. 447. It shows the pinnate form of the 

 foliage leaf but is densely covered with hairs, and chlorophyll is 

 wanting. Towards the base two to eight macrosporangia are borne 

 on the margins. It is evident that each female plant of Cycas which 

 has reached the flowering condition exhibits a regular succession of 

 flowering and vegetative periods. The flower represented by the group 

 of sporophylls is always grown through by the further development 

 of the apex which does not branch. The male plant of Cycas and the 

 other Cyeadaeeae are morphologically more advanced in that they 

 regularly produce lateral buds. Their sporophylls are borne in terminal 

 cones often of great size, while the further growth of the plant is 

 effected by a lateral bud which continues the direction of growth of 

 the sympodial axis, displacing the cone to one side. 



The cones consist of numerous oporophylls arranged spirally on 

 the axis. The microsporophylls bear large numbers of microsporangia 

 on the lower surface (Fig. 4 i8). The macrosporophylls of the cone- 

 bearing Cyeadaeeae are considerably modified as compared with Cycas, 



