620 A TEXTBOOK OF THEORETICAL BOTANY 



Anatomy of the Stem 



The characters of the stem are shown more clearly in the aerial shoots 

 than in the underground rhizomes. There are two kinds of stems in 

 Equisetiini arvense. Firstly, the fertile shoots which appear in March and 

 bear the reproductive organs. These stems are unbranched, pale brown 

 in colour, contain no chlorophyll either in their leaf-sheaths or the stem 

 itself, and die down as soon as the spores are shed. The second kind of 

 stem is much branched and forms the vegetative system and, possessing 

 chlorophyll, it serves as the assimilatory organ of the plant. The surface 

 is ribbed lengthwise, and the ribs alternate regularly in successive internodes. 

 The lateral branches arise from buds on the stem at the nodes, inside the 

 leaf-sheaths. They are equal in number to the leaves of the node at which 

 they arise and alternate with them. As the buds develop they break through 

 at the base of the leaf-sheath so that they appear to come from beneath it. 

 The initial cell of the bud divides into an upper, or epibasal, and a lower, 

 or hypobasal half, and the subsequent divisions of these two cells follow 

 the same plan as in the development of the embryo. The branch is 

 organized from the epibasal and a root from the hypobasal portion, though 

 in the aerial stem it usually remains dormant. It is a simple and primitive 

 type of shoot organisation, based on a single axis, as in the gametophytes 

 of the Musci. 



The internal anatomy of the stem is relatively simple and the vascular 

 tissues are only poorly developed. The leaf trace bundles are arranged in a 

 single circle, one entering from each leaf and passing straight down through 

 the whole length of the internode until it reaches the node below, where it 

 forks into two and joins with two bundles coming up from the internode 

 below. It follows, therefore, that every internode contains a ring of as many 

 bundles as there are leaves at the node above, and as the leaves alternate 

 with each other at successive nodes, so also do the bundles in the corre- 

 sponding internodes. Each ridge on the surface of the stem corresponds to 

 the position of a vascular bundle, therefore the ridges also alternate. 



In a transverse section through the stem we find that on the outside there 

 is a strong epidermis with stomata in the grooves (Fig. 630). These stomata 

 are peculiar in that the guard cells are completely covered externally by a pair 

 of subsidiary cells, so that a double set of guard cells, one above the other, 

 appears to be present. The cells of the epidermis are also remarkable for their 

 strongly silicified outer walls, which accounts for the extremely hard texture 

 of the stems of these plants, and earned for them their old name of " scouring 

 rushes." 



The cortex contains a variety of tissues. On the periphery, below the 

 ridges, are longitudinal ribs of sclerenchyma which give the stem mechanical 

 strength, the stelar xylem being poorly developed. Alternating with these, and 

 therefore below the grooves on the surface, are bands of assimilating tissue 

 containing chlorophyll, which take the place, physiologically, of the much- 

 reduced leaves. The inner cortex is parenchymatous and contains a series 



