MORPHOLOGY OF HIGHER PLANTS. 209 



terrupted during certain periods of the season, for instance 

 during the winter or during the dry seasons of tropical chmates. 

 And since the tissues, which are formed at the beginning of each 

 season's growth are distinct from those already formed during 

 the previous season in both color and structure of the wood (espe- 

 cially in the thickness of cell-walls and the width of the tracheae 

 or vessels), w:e perceive in this manner distinct zones of wood, 

 or the "annual rings " as they are called, the larger vessels with 

 thin walls being produced in the spring and early summer. 



Various abnormal stem-structures are known which are due 

 to certain peculiarities in the growth in thickness of stems. 

 These are especially noticeable in lianes. In some of the Mono- 

 cotyledons, as in Drac3sna, Yucca, Agave and Aloe we find a 

 secondary increase in growth of the stems. 



Plant Hairs. When the surface of the plant (either of stems 

 or leaves) is covered with short, fine hairs, which are not very 

 dense and not matted, the surface is described as pubescent; 

 when the hairs are relatively long but scattered the surface is said 

 to be VILLOUS ; or when the hairs cover each other in one direction 

 it is described as sericeous or silky. When the hairs are stiff 

 .though slender we speak of a hirsute covering; when the hairs 

 are vernate. thickish and stiff, as in Borago, the surface is spoken 

 of as being hispid. If the hairs are bristle-like the surface is 

 described as strigose ; or if they are terminated by a globular, 

 glandular head (Figs. 285, 287), as glandular. Again, when 

 the hairs arc matted the surface is described as lanate; when 

 they are long it is said to be woolly ; or when they are short 

 and soft as in Mullein it is said to be tomentose. 



When the hairs are hard and prickle-like the surface is 

 described as hispid or strigose ; when they are modified to spines 

 it is said to be spinose; and when they are hooked it is described 

 as echinate. 



In still other cases the epidermal cells, particularly of leaves, 

 are uneven, forming depressions and protuberances which if 

 slight give the surface the appearance described as rugose; or if 

 wart-like, give the appearance known as verrucose. Further- 

 more, the veins of leaves may be quite prominent, particularly 



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