144 



LECTURE IX. 



plerome-sheaths are very distinct in the rhizomes of many Monocotyledons, as 

 Iris, Acorus, and so forth ; and also in the sub-aerial shoot-axes of many Dicoty- 

 ledons. More frequently, a layer of tissue similar to the endodermis is present in 

 sub-aerial shoot-axes and leaves, either only on the outside, or also on the inside 

 of each single vascular bundle; or each individual bundle may be entirely sur- 

 rounded by such a layer of tissue, as in Fig. 153. In many other Ferns, this 

 vascular bundle-sheath is distinguished by a strong thickening of the longitudinal 

 walls turned towards the bundle, and often also by a dark brown colour (cf. 

 Fig. 136). In other cases, particularly in the upright shoot-axes and leaves of 

 many Monocotyledons, especially of the Grasses, Palms, Dracanas, etc., which require 



greater elasticity and rigidity, 



the vascular bundle-sheath 

 consists of a more or less 

 thick, often (especially in 

 Palms) extraordinarily thick 

 layer of very thick-walled, 

 lignified, long, spindle- 

 shaped fibres, which fit to- 

 gether without intercellular 

 spaces. While the function 

 of the ordinary endodermis 

 is apparently only to render 

 slow the exchange of sap be- 

 tween the parenchymatous 

 fundamental tissue and the 

 vascular bundles, these large 

 lignified fibrous strands 

 (Sclerenchyma) not only 

 serve this purpose, but upon 

 them depends, in plants not 

 properly forming wood 



FIG. 153.— Portion of the transverse section of one of the large vascular bundles of OtherWlSC, thC rigidity of 



the stem of /"/^^/i- «?!«/!««, with surrounding parenchyma />: the latter is filled (in ,1, u <■ " iV, 



winter) with starch, j spiral vessel in the focus of the elliptical transverse section ; this InS SDOOt-aXeS. SinCC tlie 



is surrounded by thin-walled wood-cells containing starch, i'^ the scalariform vessels ; i U ^1 C U 



j;> sieve-tubes. 3 bast cells with thick soft walls; Ji>- bundle sheath. Between Ä and j^^ VaSCUlar DUnOleS Ol SUCtl 



is a layer of cells containing starch. -^^ ^ i j i 



Monocotyledons, and some 

 similarly constructed Dicotyledons, in themselves very thin and feeble, are not at all 

 calculated to give the necessary rigidity to the stems and shoot-axes. These 

 sclerenchyma-sheaths, moreover, do not always surround the entire vascular bundle, 

 as in Fig. 154; frequently they accompany it only on its outer or inner side, or on 

 both. Moreover, such layers of sclerenchyma, which belong to the fundamental 

 tissue, do not always immediately accompany the vascular bundles; but very 

 frequently are quite independent from these, as layers and strands running in 

 the parenchyma of the fundamental tissue. They are particularly well seen 

 as stout, dark brown, hard bands in the stem of the Bracken-fern (Fig. 134, 

 A, pr.) as well as in the stems of Tree-ferns, where they not rarely form 

 a hard protective coat, both under the epidermis and around each of the large 



