THE ANGIOSPERMAE : STEMS 



917 



A B 



Fig. 898. — Comparison of the section of a reinforced concrete chimney (A) and a trans- 

 verse section of the stem of Trichophomm germanicum (B). The sclerenchyma 

 strands marked in black correspond to the iron rods ; the ground tissue corresponds 

 to the concrete. {After Rasdorsky.) 



Anomalous Structures 



So great is the number of Angiosperms and so infinitely various their 

 structure, that it is difiicuh to say what should be considered anomalous, 

 where the normal itself can scarcely be defined. We can only select for 

 mention a few examples of peculiarities which are non-environmental, that 

 is to say, are not directly related to the external conditions. 



1. Phloem and Xylem Strands. Separate strands of phloem only, in 

 addition to the normal collateral bundles, are not uncommon in flower and 

 fruit stalks, presumably in connection with the need for a large inflow of 

 carbohydrates to the fruit during seed ripening. Pure xylem strands are 

 less common, but they occur in the tubers of the Paeony. 



2. Medidlary Bundles. Accessory bundles, in the pith, are found in many 

 Dicotyledons, and are characteristic of a few families, such as the Piperaceae 

 (Fig. 899), members of which possess from one to four rings of such bundles 

 interior to the normal ring, which alone undergoes secondary thickening. 

 These bundles belong to the leaf traces. They descend through one inter- 

 node in the normal ring and then curve inwards and pass through another 

 internode in the pith. The two rings of bundles which are usually formed 

 in Cucurbitaceae (Fig. 900) are likewise all connected to the leaf traces. The 

 numerous medullary bundles in Begonia do not appear to become leaf traces 

 but they anastomose freely with those that do and so form part of the trace 

 system. 



Some dispute exists about the presence of truly " cauline " bundles 

 in the stems of Angiosperms. The medullary bundles in Amaranthus, for 

 example, like those in Begonia, are cauline in the sense that they are not 

 themselves actual leaf traces, but they branch from and join with leaf traces, 

 so that they are physiologically part of the leaf trace system. Whether they 



