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18

Because of a force known as geotropism - it is a reaction to gravity. The upward growth of shoots from seeds is known as negative geotropism whereas the downward growth of roots is known as positive geotropism. The act of a seed to decide which way is up, or to orient itself, is geotaxis - it detects which way is down and up, in other words, because of ...


14

It's a fascinating topic! While most of the bacteria known to colonize babies comes from the vaginal tract during birth and then later, through breastfeeding, although there is evidence to suggest that microbial colonization may begin before birth. Regarding what these bacteria are, our microbiomes are composed of probably hundreds of species of bacteria ...


14

In addition to the geotropism mentioned in the other answers, experiments on the ISS have shown that plants will grow oriented in a manner such that “upward” (i.e. the stem, leaves, etc.) is toward a light source, even in the absence of gravity. Of course, this depends on the plant being able to determine which way is toward the light source. Although I have ...


12

Short answer Large animals do get cancer. They may contract cancer with an incidence less than that estimated by absolute cell numbers, but there seems to be a lack of data on cancer rates in large animals to support this hypothesis conclusively. Background Whales contract cancer (Martineau et al, 2002). There does, however, seem to be a lack of correlation ...


10

@JM97 is correct. According to National Geographic >> The gestation period of a Red Kangaroo is 33 days and it's baby at birth weighs a mere 0.03 oz. To put it into a more precise perspective, on birth, the baby weighs 1/100,000 of an adult red kangaroo's weight. Second place goes to the baby of a Giant Panda; at birth it weighs 3.2oz which is 1/900 of ...


8

There were some experiments done in microgravity in longer space shuttle missions. The reports show that the fungi develop relatively normal but grow in random orientations instead of orientating upwards. See this images: The upper image shows fungi grown on earth which are subjected to normal gravity. The lower image shows fungi (actually only the fruiting ...


7

Lightning results in nitrogen fixation (Draphco et al., 1967; Hill et al., 1980). It is one of the main abiotic nitrogen fixation mechanisms. As you would know very well, nitrogen fixation is important for all organisms.


7

I assume by asking why, you are asking about the distal evolutionary causes, and not the molecular mechanisms that account for these things. (Important disclaimer: these causes are difficult to be certain about; they require a fair amount of informed speculation.) With that said: it is widely agreed upon in evolutionary biology that human males, as in ...


6

Short answer: Yes, we do shrink with age. The most important reason is that the cartilage in the joints between our bones gets worn out and thinner, as well as disks between the vertebrae of the spine. This results in a compression of the spine and also to a loss in height. Shrinking bones due to osteoporosis can also play a role, as well as muscular ...


6

Considering your assumption: I'm just looking at the exponential part, where the simple exponential equation works. If we assume there's sufficient nutrients for bacteria to grow unchecked for a number of hours (more-or-less true in a real culture) In your original model you are using discrete states and fixed time steps. So, if 30 min is one time ...


6

Mineral nutrients are conveyed up the tree via the xylem by transpiration. The phytohormone cytokinin that releases buds/shoots is synthesized in the root tips and is also conveyed via the transpiration stream. Transpiration is driven by the water potential of the air --> ln(rH); transpiration stops when the relative humidity is 100%. Even though some ...


6

Simple Answer: There is no simple answer, because transpiration is just one of many factors that affect plant growth, and even in controlled experiments, the conclusions deviate according to different species. Background: First of all, we know that increase in atmospheric humidity leads to less transpiration. A graph between transpiration and humidity, as ...


5

Oxygen is good for animals because our basic metabolism is this: High energy carbon molecules + O2 → energy + H2O + CO2 Plants do that too at night, but during the day, they mostly do this: High energy photons + H2O + CO2 → High energy carbon molecules + O2 Rubisco, one of the most important enzymes in photosynthesis, can bind to O2, leading to less ...


5

The answer is probably No. "Long" bones - like the tibia, fibula, femur, humerus, etc. grow at the ends during childhood via a special formation called the Epiphyseal plate (also known as "growth plates"). The plates are composed of a special cartilage that grows, and slowly calcifies as a person reaches adulthood. When the growth plates completely stop ...


5

Short answer Neural regeneration in the adult brain is confined to two localized hot spots, namely the hippocampus and the anterior lateral ventricular wall. Establishing connectivity between new cells and existing neurons is mediated, at least in part, by the growth cones present on axonal tips of newly formed cells. The growth cone responds to various ...


5

The question is a bit vague but I will take it to mean the following Why does it (the body) stop in terms of height or physical mass when it can still keep on growing? The answer is physics, specifically the ability for a body of a specific shape and structure to support itself and move itself, followed by the energy requirement to feed all those cells. ...


5

More details to support @SPr's answer, from the abstract of Plavcan (2012): "Sexual Size Dimorphism, Canine Dimorphism, and Male-Male Competition in Primates: Where Do Humans Fit In?" While dimorphism in primates is associated with agonistic male mate competition, a variety of factors can affect male and female size, and thereby dimorphism. The causes of ...


4

The answer to your question depends on many variables. To narrow down the list of possible species it might be helpful to list under which conditions you intend to grow this algae and whether you want to give an additional carbon source such as glucose or acetate. To give you an idea why this is important: the fastest growing alga I have ever worked with is ...


4

I cannot say anything about the general case, or specifically for day-neutral plants. However, Sforza et al. (2012) have studied the effects of light conditions on algae (used for biofuel production), and their results indicate several problems with continuous light. In continuous light conditions they find lower chlorophyll contents and higher carotenoid ...


4

If $W$ is a parameter, I'd expect (in the simplest case) that $W$ itself is a constant (i.e. it doesn't have a distribution). (By the way, I would normally use $T_{1/2}$ to denote the doubling time, $\ln 2/\ln W$, and would use $\lambda = \ln W$ for the exponential rate, i.e. $W^t= \exp(\ln W)^t = \exp(t \ln W) = \exp(\lambda t)$. I don't normally see people ...


4

Check out Kendall (1949), section 2. He shows that the pdf is a geometric distribution. In your notation, it's $P_n(t) = N(0)W^{-t}\left(1-W^{-t}\right)^{n-1}$, which implies that the mean is indeed $E[N(t)] = N(0)W^t$ and the variance is $\text{Var}[N(t)] = N(0) W^t(W^t-1)$. (Be careful -- his definition of $\lambda$ differs from yours by a factor of $\ln 2$...


4

The transport of water, and dissolved minerals, from the soil to the tops of trees is a combination of three factors. Root pressure pulls the water in due to factors related to osmosis. Capillary action helps draw it up the hydrophyllic vascular tissue. But the most important factor is due to evaporation in the leaves. This has some rather detailed aspects, ...


4

The National Cancer Institute (NCI) has a list of doubling times for a variety of commonly used cell lines, see https://dtp.cancer.gov/discovery_development/nci-60/cell_list.htm I my experience, these doubling times are quite stable/reproducible, provided that cells are kept in standard culture conditions. While doubling times differ a lot between cell ...


4

I don't know what you mean with 'used on' but gibberellins are plant hormones in ferns, monocots and dicots. All of these plants will react to the application of gibberellin but the effect will depend on the dose, type of molecule and tissue where it is applied. All plant hormones have multiple effects but the classic one for gibberellic acid is stem ...


4

This is not the best answer, but see that as a starter! Feel free to use my answer to build yours! I have found a so-called professor speaking of advantages to go on a 24h light/12h dark cycle (36h cycle) to accelerate the growth of cannabis. I'm not sure of the reliability of this source. I think the main problem we will encounter with that reduction ...


3

Muscle growth and development is mediated by "trophic factors" not biochemical reactions (ATP) or muscle contraction directly. The best method of releasing tropic factors is exercise and not electrical stimulation or biochemical reactions (ATP) or biomechanical contraction. Nerve ending release growth or tropic factors which stimulate muscle growth and for ...


3

Human bones grow in length due to the effects of human growth hormone on the epiphyseal plates near the ends of the bone. Over time these two growing margins decrease in size, or width, until those special cells are either gone, or can no longer respond to hGH. At that point the body has no mechanism to increase the size of the skeleton, and growth stops. In ...


3

Certain conditions are necessary for seed germination: Moisture — seeds dry out during dispersal, but at germination the cells need to absorb water in order to rehydrate so that metabolic reactions can take place, which will eventually lead to the growth of the embryo tops and shoot. Furthermore, some seeds contain a hormone that inhibits germination and ...


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