THE TISSUES OF THE STEM 67 



would be cut off entirely from the outer air, were it not for interrup- 

 tions of the continuity here and there. These are called lenticels, 

 and they may be seen with the naked eye on most stems, as brownish, 

 slightly swollen spots (Fig. 6, p. 13). A lens shows that here the 

 epidermis is split, and that powdery tissue lies within. Microscopi- 

 cally it is seen that in place of the closely fitting cork-cells those of the 

 lenticel are rounded, with intercellular spaces, so that the tissue is 

 spongy, and allows ventilation into the cortex (Fig. 44, /.). The 

 lenticels remain for years, and may grow to a large size, as may be 

 seen on the surface of many woody trunks, where they often 

 determine the position of the fissures of the bark. The brown 

 crumbling spaces in bad bottle-corks are the lenticels, which traverse 

 the otherwise impervious cork of the Cork-Oak. 



The stem of a Dicotyledon showing secondary thickening as thus 

 described is mechanically a stable structure. Its form is that of a 

 cone with its base at the level of the ground. There it often widens 

 out into a broad " stool," which helps to give it stability. Many 

 large trees of the tropics form radiating buttresses at their base, 

 which are still more effective. The stems of Monocotyledons are 

 constructed differently. Most of them do not increase in bulk at the 

 extreme base ; but developing stronger above than below, assume the 

 form of an inverted cone, with its apex at the level of the soil. 

 This unstable structure is propped up by roots, which act like oblique 

 struts ; this may be very clearly seen in large plants of the Maize, or 

 of the Screw Pine. In many large Palms, however, the base of the stem 

 distends with age. This is due to a general expansion of the con- 

 junctive parenchyma, which may be accompanied by the formation 

 of additional deeply-seated vascular strands. In some few Mono- 

 cotyledons, however, as in Dracaena, there is a cambial increase. It 

 arises in the pericycle, outside the primary bundles, and it forms 

 new vascular strands which are closed, and embedded in a sclerotic 

 parenchyma. Physiologically the result is the same as in Dicoty- 

 ledons, but it arises in a different manner (see Chap. XXXVI, p. 590). 

 Those who have followed the foregoing description of the woody 

 Dicotyledons will see how admirably the trunk meets the requirements. 

 Their shoot-system is constructed on a scheme of indefinite expansion, 

 consequent on continued apical growth and branching. The demands 

 that will be made upon the trunk and branches are well illustrated 

 by any Sycamore or Beech tree exposed to a wind in early summer, 

 after the leaves are expanded and the tree is in full flower. The strain 

 of the wind-pressure is transferred from the leaves to the twigs, and 



