602 PHYSIOLOGY. 



dermis, and are therefore to be regarded as trichomes, while others con- 

 sider them to originate beneath the epidermal layer like phyllomes. 

 Warming's view is that the ovules originate in the periblem, the derma- 

 Fig. 601. 



Development of ovule : a, primary nucleus, invested at b by the primine, and this by the 

 secundine (c) ; at d the ovule has become anatropoua. 



togen furnishing the coats. The embryo-sac is formed by a cell of the 

 first layer of periblem. The ovules thus originate in the same layer of 

 tissues as do true buds. The coats of the ovule are probably foliar, the 

 nucleus axial. 



The structure and mode of development of the pollen and of the ovules 

 will be farther alluded to under the head of Physiology of the Eepro- 

 ductive Organs. 



Buds. These are the growing points enveloped by the lateral 

 growths, which, growing faster, eventually cover over the point 

 from which they sprung. Like hairs and leaves, they are exo- 

 genous, or formed by lateral outgrowths from a superficial cell or 

 mass of cellular tissue (periblem and derrnatogeu), and are not, 

 like roots, endogenous, or formed in the interior of the tissues. 



Adventitious buds, however, are formed, like roots, in an endo- 

 genous manner, as also are the shoots of Equisetacese. It is re- 

 quisite, however, not to confound adventitious buds with latent buds, 

 which are truly exogenous and normal, save that their development 

 is temporarily or permanently arrested. 



According to Warming, the bud and the leaf in whose axil it is 

 placed are organically united, and may be taken as parfcs of one 

 whole, the two parts being developed to an equal degree, or dif- 

 ferently, according to the functions they have to fulfil ; one may 

 exist without the other, or both may be present. According to this 

 view, the parent leaf is not a primary stage belonging to the stem, 

 and the axillary bud a secondary phase of growth ; but the parent 

 leaf is really the first leaf of the 'bud itself, and of the same stage 

 or degree of growth as it. 



Shoots are only the developed buds. Their mode of ramification 

 has been before alluded to (p. 38). The only point that need be 

 here alluded to is the ramification by partition, in which the cells 

 of the growing-point actually divide into two or more groups in a 

 dichotomous or polytomous manner, as in Hydrocharis, the tendrils 

 of Vitis vulpina, the scorpioid cymes of some Boragmaceae, Ascle- 



