MINDTE STEUCTUEB. 



69 



The general structure of the steins of orchids will be readily understood 

 from the annexed sections made from four species well known in 

 cultivation. The development of a stem of a monopodial orchid is 

 shown by transverse sections of the stem of Va7ida tricolor (Figs. 14 

 and 15) at three successive periods of growth, and to give a clearer 

 idea of the form of the individual cells fragmentary sections both 

 transverse and longitudinal are given of the stem of CaUleya infermeih'a 

 enlarged 200 diameters (Fig. 16). 



Roots. — The general structure of the roots of orchids is described 

 under the vegetative organs. As regards the minute structure the main 



S-tk! 



Fig. 13. Transverse section of stem of Lwlia imrpurata. A, enlarged 2 diameters. B, a fragment of the same 



enlarged 40 diameters. 



1 , epidermis ; 2, hypodermal layers of thick-walled cells ; 3, fundamental tissue ; 4, air cavities in the 

 fundamental tissue : 5, fibro-vascular bundles or woody fibres of the stem. 



features are the same as those of other monocotyleclouous plants. There 

 is (1) an epidermis, not always well detined, and beneath this (2) a 

 band of cortical tissue, Avhich is much more developed in some genera 

 than in others, and in aerial more than in terrestrial roots ; in the 

 former it consists of comparatively large thin-walled cells filled with water, 

 and often with a fine spiral thread coiled within them. Underlying 

 the cortical tissue is a layer of thick-walled cells (3) called the 

 endodermis enclosing (4) the fundamental tissue of the root; and lastly 

 there is (5) the axial cylinder consisting of bast and wood with ligneous 



