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698 A TEXTBOOK OF THEORETICAL BOTANY 



where it is quite extensive and has a separate protoxylem, distinct from that 

 of the centrifugal xylem. The latter feature recalls the similar structure 

 in the sporophyll of Cycas and may be perhaps looked upon as a link with 

 the Cycads. 



On the other hand the leaf traces are consistently single, not double, 

 which, in the light of comparison can only be regarded as an advanced 

 character and remote from any suggestion of Cycadean affinities. 



Resin canals of the Pinus type are altogether absent from this group. 

 Indeed Taxus has no resin glands of any kind, though in the other two genera 

 elongated resin sacs occur, especially in the leaves and the flower parts. 



The characters of the Taxales make up a rather perplexing whole. They 

 certainly stand well apart from the Coniferales, their nearest living link 

 being possibly to Ginkgo. Geologically their remains do not go back beyond 

 the Cretaceous period, so that it is impossible to connect them directly with 

 any Palaeozoic group, but their morphological peculiarities, their small 

 numbers and their discontinuous geographical distribution all seem to 

 indicate that they are a group which has had an ancient history and is 

 now decadent. 



The possibilities are in favour of the derivation of Taxales from the same | 

 ancestral stock as the Coniferales, but they must have separated from the 

 latter at an early date and have preserved some features of the ancestral stock, 

 presumably the Cordaitales, which have since been lost by the Conifers. 



Taxus baccata (The Yew) 



There is only one species of Taxus, though there are a number of sub- 

 species, some of which range farther than the type, several occurring in North 

 America and one or two in Japan. 



It is a slow-growing, evergreen tree, seldom reaching a height of more than 

 30 ft. (Fig. 700), although specimens estimated to be 3,000 years old are 

 known, the oldest of which, a mere hollow shell, stands in the churchyard 

 of Fortingal in Perthshire, Scotland. Molisch has, however, shown good 

 reason for believing that these estimates of age are exaggerated, the enormously 

 thick trunks having probably originated by the fusion of several small ones. 



No foliar spurs are formed, and the branches grow horizontally and 

 form a very dense canopy. The leaves are 2 to 3 cm. long, narrow and obtuse, 

 and are borne spirally, but are twisted at their bases so that they lie in two 

 rows along the horizontal branches. In certain varieties with upright 

 branching the leaves are disposed all round the shoots, so that the two-ranked 

 arrangement in the normal form is apparently a response to the horizontal 

 growth of the branches. 



Anatomically Taxus conforms to the general type of Pinus, with certain 

 minor differences. There are no resin glands of any kind and the tracheids 

 of the secondary wood show a spiral marking, in addition to bordered pits 

 in a single series, as in Pinus. A single vascular bundle supplies the leaf, 

 forming a midrib, flanked on each side by transfusion tissue (Fig. 701). 



