Embryogenesis A physiological process leading to the formation of embryos.

Endocarp The usually woody-textured inner layer of the pericarp.

Endosperm 1. The multicellular food-storing tissue formed inside a seed of flowering plants following the double fertilization of the embryo sac by the second sperm nucleus. 2. The prothallus of the female gametophyte of gymnosperms.

Epicarp The outer layer of the ovary wall, the superficial layer of the pericarp especialy when it can be stripped off as a skin, the epicarpium.

Epidemiology The study of the dynamics of disease in populations. Generally has two aspects:
'Genetical' epidemiology in which the patterns of gene-flow in interacting populations of hosts and pathogens are studied. For example, a plant breeder may introduce a gene for resistance to disease. The resistance gene is first identified and evaluated in small experimental plots, and if found promising, it is then submitted to larger-scale field trials and subsequent release for commercial cultivation. The expression of disease in the host population carrying the new gene reflects the dynamic balance between adaptation and selection as the new host resistance gene exerts selection pressure on the pathogen population to try and adapt to overcome the newly introduced resistance gene. At first the pathogen population is at a disadvantage, facing a new situation (selection pressure) to which the pathogen had not been exposed previously. Consequently, most individuals in the pathogen population are not 'fit' and are epidemiologically 'incompetent' to cause disease. An observer seeing the field at this time will believe that the resistance mechanism introduced by the plant breeder is sucessful. However, since a pathogen population can pass through many hundreds more life cycles than its average host, and because the rate of adaptation is elicited and determined by the rate and nature of selection pressure, the pathogen population becomes progressively 'used to' the resistance gene by overcoming it.
Resistance genes can exert either qualitative or quantitative selection pressure. Just as there is, in chemistry, both (a1) [enzyme x substrate] specifcity; i.e. a qualitative lock-and-key recognition whereby a specific enzyme has its 'own' or a 'preferred' substrate, and (b1) quantitative variation in the rate at which enzymes after recognition, may catalyze a substrate, so too in plant disease, do (a2) vertical resistance genes in the host exert qualitative selection pressure that is usually, but not always or axiomatically more easily overcome by the pathogen; and (b2) do {{horizontal resistance}} genes exert quantitative selection pressure that is usually, but not always or axiomatically less easily overcome by the pathogen. Thus, the expression and erosion of vertical resistance may be compared to an athlete facing a pole vaulting hurdle: there is a single threshold that has to overcome after which there are no further obstacles. In contrast, the expression of horizontal resistance could be compared to mountain climbing in which every step of the route poses a small portion of the overall 'quantity' of the challenge that has to beovercome to succeed.
Host genes and pathogen genes do not interact directly, but do so through their intermedaries enzymes, proteins and other compounds determined by gene expression processes. Many of these are influenced by the environment, as are levels of pathogen population affected by external factors such as competition and, in agriculture, by factors such as crop spacing that influences micro-climate. This host x pathogen x environment (Hg x Pg x E) interaction, as it is viewed or interpreted from the perspective of genetics and plant breeding (hence the 'g' in (Hg x Pg x E) above, constitutes the 'genetical' component of epidemiology.
'Ecological epidemiology on the other hand, studies fluctuations in disease in populations due to ecological and abiotic factors that influence the host x pathogen interaction. Here too the (H x P x E) interaction is the centre of focus, but now the emphasis is on the many factors in the environment, such as weather effects, competition, density dependence etc., are the focus of study.
Ecological epidemiology is concerned with methods and procedures that measure the rates of disease in populations. It tries to identify the critical parameters; e.g. the {{incubation period}} and the latent period as well as abiotic factors such as relative humidity, rainfall, soil pH and others, that determine these rates, and then it tries to partition and ascribe the observed variation in in disease patterns to understand epidemics in terms of driving parameters and factors. The final purpose being the practical objective of reducing disease impact wthout causing environmental damage.

Epidermis The thin external layer of protective cells of a plant ; a sheath of closely united cells, usualy one cell thick, which forms a layer over the stems and leaves. It is usualy continuous except where stomata perporate it. See illustrations: leaf cross section, morphology of leaves, changes in a stem as it increases in age or primary plant body of a root.