870 



A TEXTBOOK OF THEORETICAL BOTANY 



Secretory hair glands may be simply swollen cells which are usually 

 water stores ; or they may be compound, with long or short stalks bearing 

 a head consisting of either one large secretory cell, or a more or less peltate 

 group of cells. Some of these types are illustrated in Volume IV in 

 connection with insectivorous plants. 



Prickles are classified as trichomas, no matter how massive they may 

 be, whenever it is clear that they arise from the epidermis and are not 

 modifications of any other organ. As a matter of fact the large prickles of 

 Rosa and the vascular prickles on the fruit of Horse Chestnut are connected 

 with simple hairs by all gradations of finer prickles. Prickles of all grades may 

 even be present together on the same stem, as in the Burnet Rose (Rosa 



Endodermis 



Phloem 

 Xylem 



Fig. 855. — Convallaria mojalis. Transverse section of rhizome 

 showing the broad cortex and well-marked stele with 

 endodermis. 



spinossisshna), and it is notable that in some Roses the prickles at the nodes 

 are often much larger and stronger than the others. The so-called " endo- 

 genous prickles " on the stems of some Palms are, however, really modified 

 adventitious roots and are therefore properly classified as thorns. The very 

 large and prominent spines of the Cacti are of doubtful nature, having been 

 interpreted both as modified leaves and as trichomes. They grow from 

 little humps, called areolae, from which the lateral buds, if any, usually arise, 

 and the balance of evidence points to the spines being really trichomes. 



3. The Cortex. Between the epidermis and the stele lies the zone of the 

 parenchymatous cortex. In stems it is rarely as broad as in the root, for the 

 vascular tissues, probably for mechanical reasons (see p. 912) lie much nearer 

 to the periphery than in roots. The underground stems of Monocotyledons 

 usually have a fairly broad cortex (Fig. 855), but monocotyledonous aerial 

 Stems, such as that of the Maize (Fig. 856), so often used as an anatomical 



