446 ELEMENTARY BIOLOGY. [CHAP. 



colour ; but the thickening has taken place unequally, so as 

 to leave short, obliquely directed, thin places, which look 

 like clefts and are called pits. This tissue is termed sderen- 

 chyma. The yellow bands, lastly, are vascular bundles* or 

 cylinders. Each cylinder is surrounded on the outside by a 

 layer of rather thick-walled, elongated, parallel-sided cells, 

 constituting the endodermis. The cylinder itself consists of 

 two main parts ; a central portion, constituting the xylem or 

 wood, and a peripheral portion which is the phloem or bast. 

 The xylem consists chiefly of vessels, many of them of 

 relatively large size. They are derived from cells, the 

 transverse walls between which have been partially broken 

 down. In the mature vessels the protoplasmic contents 

 have entirely disappeared; they only contain water or air. 

 Their walls are greatly thickened, the thickening having 

 taken place along equidistant transverse lines, the thin 

 spaces left between them being the pits. The vessels have 

 become flattened against one another, by mutual pressure, 

 so that they are five- or six-sided ; and, as the markings of 

 their flattened walls simulate the rounds of a ladder, they 

 have been termed scalariform vessels. The cavities of these 

 scalariform vessels are divided at intervals, in correspondence 

 with the lengths of the cells of which they are made up, by 

 oblique, often perforated partitions. In most Ferns there is 

 no such perforation. Among the smaller vessels, a few will 

 be found, in which the thickening forms a closely wound 

 spiral. These are spiral vessels. They usually occur in two 

 groups, and as they are the first elements of the xylem to 

 be differentiated, they are said to constitute the protoxylem 



1 These "vascular bundles" in the Fern do not correspond to the 

 structures of the same name, to be described below in the Broad-bean. 

 Each "bundle" in the Fern-stem is equivalent to the whole "central- 

 cylinder " of the Broad -bean, or of the Fern-ratf . 



