THE GYMNOSPERMAE : CYCADALES, ETC. 753 



wood contains none at all. Each annual ring of wood is therefore marked 

 by the limited distribution of the vessels in a narrow band, this constituting 

 the type of wood structure called ring porous. The pits on the end walls of 

 the vessel elements are at first ordinary bordered pits, but the border and 

 the closing membrane are absorbed at an early stage of development, so that 

 perforations are left which often become confluent into irregular apertures, 

 leaving open connections between successive vessel elements. There is no 

 complete absorption of the end walls, as happens in the higher Angiosperms, 

 but parallels to this partial absorption can be found in many Angiosperms 

 belonging to what we consider " lower " groups such as the Magnoliaceae. 



At or near the nodes the pith cells become strongly lignified, forming 

 transverse plates. It is in these regions that the winter abscission of young 

 branches occurs. 



There are no resin ducts, but cells with large stellate crystals of Calcium 

 oxalate are abundant. 



In the third or fourth year a phellogen arises just outside the phloem 

 and the periderm formed from it displaces all the outer tissues. 



The meristems of Ephedra are poorly differentiated into distinguishable 

 layers. In the stem apex even the surface layer is not clearly separated from 

 the central mass of meristem cells, which differentiates directly into the 

 respective permanent tissues. 



Reproduction. 



Both male and female reproductive organs are borne on small strobili, 

 the scales of which are arranged in decussate pairs, each pair being concrescent 

 at the base. These strobili are normally axillary on young shoots, either 

 singly or in small sessile groups, which really represent suppressed branches. 



There are certain problems of morphological interpretation regarding 

 these strobili which will be discussed briefly later. 



Ephedra is normally dioecious, but monoecious plants and even herma- 

 phrodite flowers are known. Bisporangiate strobili produce stamens at the 

 base and ovules above, as in the similar cases in Coniferales, 



The Male Strobilus. 



Each male strobilus stands in the axil of a leaf and each has a short axis 

 bearing a number of rather thick and closely set scales or bracts, arranged 

 decussately in pairs (Figs. 755 and 756). The number of pairs varies between 

 two and twelve. In the axil of each bract stands a single male flower (Fig. 763), 

 which consists of a perianth of two scales, from between which arises a 

 stamen, consisting of a stalk, at the top of which is a variable number of 

 anthers. These are normally sessile, but in species where the anthers have 

 individual filaments they make the stalk appear to branch apically. 



The nature of the stalk or antherophore is uncertain. It may be either 

 an upgrowth of the flower base or formed by a fusion of anther filaments. 



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