274 ANGIOSPERHE. 



Our knowledge, however, is still so incomplete that one would 

 hesitate to accord the full systematic value which Dr. Treub 

 attaches to his discovery until the limits of the Chalazogamic 

 group are better denned ; and it would hardly be justifiable to 

 include the Casuarinas and the above-noted genera in one 

 family.] 



Class 1. Monocotyledones. 



The embryo has only one cotyledon ; the leaves are as a rule scattered, 

 with parallel venation ; the vascular bundles of the stem are closed, 

 there is no increase of thickness. The flower is typically constructed of 

 five 3-merous whorls, placed alternately. 



THE EMBRYO is generally small in proportion to the abundant 

 endosperm (exceptions, see Helobiece), and its single cotyledon is 

 often sheath-like, and very large. On the germination of the 

 seed either the entire cotyledon, or its apex only, most generally 

 remains in the seed and absorbs the nutritive-tissue, while the 

 lower portion elongates and pushes out the plumule and radicle, 

 which then proceed with their further growth. The primary root 

 in most cases soon ceases to grow, but at the same time, however, 

 numerous lateral roots break out from the stem, and become as 

 vigorous as the primary root, or even more so. Increase in 

 thickness does not take place in these roots ; they branch very 

 little or not at all, and generally die after a longer or shorter time. 



THE STEM is frequently a corm, bulb, or other variety of under- 

 ground stem, as the majority of the Monocotyledons are perennial, 

 herbaceous plants ; it has scattered, closed vascular bundles (Fig. 

 276), and no cambium by which a continuous thickening may take 

 place. The stem of the Palms, however, attains a very consider- 

 able thickness, which is due to the meristem of its growing-point 

 continually increasing in diameter for a lengthened period (often 

 for many years), until it has reached a certain size. In this con- 

 dition the growing-point has the form of an inverted cone, and it 

 is only when this cone has attained its requisite size that the for- 

 mation of a vertical cylindrical stem commences. Certain tree- 

 like Liliaceas, as Draccena, Aloe, etc., have a continuous increase in 

 thickness ; this is due to a meristematic layer, which arises in the 

 cortex, outside the original vascular bundles, which were formed at 

 the growing-point of the stem. This meristem continues to form 

 thick-walled parenchyma and new, scattered vascular bundles. 

 The primary vascular bundles, in the Palms and others, run in a 



